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Unicode, formally The Unicode Standard,The formal version reference is is an
information technology Information technology (IT) is the use of computers to create, process, store, retrieve, and exchange all kinds of data . and information. IT forms part of information and communications technology (ICT). An information technology syste ...
standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems. The standard, which is maintained by the Unicode Consortium, defines as of the current version (15.0) 149,186 characters covering 161 modern and historic
scripts Script may refer to: Writing systems * Script, a distinctive writing system, based on a repertoire of specific elements or symbols, or that repertoire * Script (styles of handwriting) ** Script typeface, a typeface with characteristics of handw ...
, as well as symbols, emoji (including in colors), and non-visual control and formatting codes. Unicode's success at unifying character sets has led to its widespread and predominant use in the internationalization and localization of computer
software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consist ...
. The standard has been implemented in many recent technologies, including modern
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
s,
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
, and most modern programming languages. The Unicode character repertoire is synchronized with ISO/IEC 10646, each being code-for-code identical with the other. ''The Unicode Standard'', however, includes more than just the base code. Alongside the character encodings, the Consortium's official publication includes a wide variety of details about the scripts and how to display them: normalization rules, decomposition, collation, rendering, and bidirectional text display order for multilingual texts, and so on. The ''Standard'' also includes reference data files and visual charts to help developers and designers correctly implement the repertoire. Unicode can be stored using several different encodings, which translate the character codes into sequences of bytes. The Unicode standard defines three and several other encodings exist, all in practice variable-length encodings. The most common encodings are the
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
-compatible UTF-8, the ASCII-incompatible UTF-16 (compatible with the obsolete UCS-2), and the Chinese Unicode encoding standard GB18030 which is not an official Unicode standard but is used in China and implements Unicode fully.


Origin and development

Unicode has the explicit aim of transcending the limitations of traditional character encodings, such as those defined by the ISO/IEC 8859 standard, which find wide usage in various countries of the world but remain largely incompatible with each other. Many traditional character encodings share a common problem in that they allow bilingual computer processing (usually using Latin characters and the local script), but not multilingual computer processing (computer processing of arbitrary scripts mixed with each other). Unicode, in intent, encodes the underlying characters— graphemes and grapheme-like units—rather than the variant glyphs (renderings) for such characters. In the case of Chinese characters, this sometimes leads to controversies over distinguishing the underlying character from its variant glyphs (see Han unification). In text processing, Unicode takes the role of providing a unique —a
number A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual number ...
, not a glyph—for each character. In other words, Unicode represents a character in an abstract way and leaves the visual rendering (size, shape, font, or style) to other software, such as a
web browser A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used o ...
or
word processor A word processor (WP) is a device or computer program that provides for input, editing, formatting, and output of text, often with some additional features. Early word processors were stand-alone devices dedicated to the function, but current ...
. This simple aim becomes complicated, however, because of concessions made by Unicode's designers in the hope of encouraging a more rapid adoption of Unicode. The first 256 code points were made identical to the content of ISO/IEC 8859-1 so as to make it trivial to convert existing western text. Many essentially identical characters were encoded multiple times at different code points to preserve distinctions used by legacy encodings and therefore, allow conversion from those encodings to Unicode (and back) without losing any information. For example, the " fullwidth forms" section of code points encompasses a full duplicate of the Latin alphabet because Chinese, Japanese, and Korean ( CJK) fonts contain two versions of these letters, "fullwidth" matching the width of the CJK characters, and normal width. For other examples, see
duplicate characters in Unicode Unicode has a certain amount of duplication of characters. These are pairs of single Unicode code points that are canonically equivalent. The reason for this are compatibility issues with legacy systems. Unless two characters are canonically equi ...
. Unicode Bulldog Award recipients include many names influential in the development of Unicode and include Tatsuo Kobayashi, Thomas Milo,
Roozbeh Pournader Roozbeh Pournader( fa, روزبه پورنادر, ) is a free software activist and expert on Unicode text encoding, text rendering, and fonts, especially for bidirectional text. He is originally from Iran, and is now living in the United Sta ...
,
Ken Lunde Ken Roger Lunde (, born 12 August 1965 in Madison, Wisconsin)Lunde, 2008. is an American specialist in information processing for East Asian languages. Academic Background Ken majored in linguistics at University of Wisconsin–Madison in 1985, ...
, and Michael Everson.


History

Based on experiences with the
Xerox Character Code Standard The Xerox Character Code Standard (XCCS) is a historical 16-bit character encoding that was created by Xerox in 1980 for the exchange of information between elements of the Xerox Network Systems Architecture. It encodes the characters required f ...
(XCCS) since 1980, the origins of Unicode can be traced back to 1987, when Joe Becker from Xerox with Lee Collins and
Mark Davis Mark Davis may refer to: Entertainers *Mark Davis (talk show host), American radio talk show host *Mark Jonathan Davis (born 1965), American actor/singer and creator of Richard Cheese *Mark Davis, American bassist and founding member for the band ...
from
Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...
started investigating the practicalities of creating a universal character set. With additional input from Peter Fenwick and
Dave Opstad David G. Opstad (born ) is a retired American computer scientist specializing during his career in computer typography and information processing (focusing on character encodings), leading to several breakthroughs. Opstad was a contributor to U ...
, Joe Becker published a draft proposal for an "international/multilingual text character encoding system in August 1988, tentatively called Unicode". He explained that "the name 'Unicode' is intended to suggest a unique, unified, universal encoding". In this document, entitled ''Unicode 88'', Becker outlined a 16-bit character model:
Unicode is intended to address the need for a workable, reliable world text encoding. Unicode could be roughly described as "wide-body
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
" that has been stretched to 16 bits to encompass the characters of all the world's living languages. In a properly engineered design, 16 bits per character are more than sufficient for this purpose.
His original 16-bit design was based on the assumption that only those scripts and characters in modern use would need to be encoded:
Unicode gives higher priority to ensuring utility for the future than to preserving past antiquities. Unicode aims in the first instance at the characters published in modern text (e.g. in the union of all newspapers and magazines printed in the world in 1988), whose number is undoubtedly far below 214 = 16,384. Beyond those modern-use characters, all others may be defined to be obsolete or rare; these are better candidates for private-use registration than for congesting the public list of generally useful Unicodes.
In early 1989, the Unicode working group expanded to include Ken Whistler and Mike Kernaghan of Metaphor, Karen Smith-Yoshimura and Joan Aliprand of RLG, and Glenn Wright of Sun Microsystems, and in 1990, Michel Suignard and Asmus Freytag from Microsoft and Rick McGowan of NeXT joined the group. By the end of 1990, most of the work on mapping existing character encoding standards had been completed, and a final review draft of Unicode was ready. The Unicode Consortium was incorporated in California on 3 January 1991, and in October 1991, the first volume of the Unicode standard was published. The second volume, covering Han ideographs, was published in June 1992. In 1996, a surrogate character mechanism was implemented in Unicode 2.0, so that Unicode was no longer restricted to 16 bits. This increased the Unicode codespace to over a million code points, which allowed for the encoding of many historic scripts (e.g., Egyptian hieroglyphs) and thousands of rarely used or obsolete characters that had not been anticipated as needing encoding. Among the characters not originally intended for Unicode are rarely used Kanji or Chinese characters, many of which are part of personal and place names, making them much more essential than envisioned in the original architecture of Unicode. The Microsoft TrueType specification version 1.0 from 1992 used the name 'Apple Unicode' instead of 'Unicode' for the Platform ID in the naming table.


Unicode Consortium

The Unicode Consortium is a nonprofit organization that coordinates Unicode's development. Full members include most of the main computer software and hardware companies with any interest in text-processing standards, including Adobe,
Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...
, Facebook,
Google Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
, IBM, Microsoft,
Netflix Netflix, Inc. is an American subscription video on-demand over-the-top streaming service and production company based in Los Gatos, California. Founded in 1997 by Reed Hastings and Marc Randolph in Scotts Valley, California, it offers a ...
, and SAP SE. Over the years several countries or government agencies have been members of the Unicode Consortium. Presently only the
Ministry of Endowments and Religious Affairs (Oman) The Ministry of Awqaf and Religious Affairs (MARA) is the governmental body in the Sultanate of Oman responsible for overseeing all matters related to awqaf and religious affairs. The current Minister of Awqaf and Religious Affairs is Abdullah ...
is a full member with voting rights. The Consortium has the ambitious goal of eventually replacing existing character encoding schemes with Unicode and its standard Unicode Transformation Format (UTF) schemes, as many of the existing schemes are limited in size and scope and are incompatible with multilingual environments.


Scripts covered

Unicode currently covers most major writing systems in use today. , a total of 161
scripts Script may refer to: Writing systems * Script, a distinctive writing system, based on a repertoire of specific elements or symbols, or that repertoire * Script (styles of handwriting) ** Script typeface, a typeface with characteristics of handw ...
are included in the latest version of Unicode (covering alphabets,
abugida An abugida (, from Ge'ez: ), sometimes known as alphasyllabary, neosyllabary or pseudo-alphabet, is a segmental writing system in which consonant-vowel sequences are written as units; each unit is based on a consonant letter, and vowel no ...
s and
syllabaries In the linguistic study of written languages, a syllabary is a set of written symbols that represent the syllables or (more frequently) moras which make up words. A symbol in a syllabary, called a syllabogram, typically represents an (optiona ...
), although there are still scripts that are not yet encoded, particularly those mainly used in historical, liturgical, and academic contexts. Further additions of characters to the already encoded scripts, as well as symbols, in particular for mathematics and music (in the form of notes and rhythmic symbols), also occur. The Unicode Roadmap Committee ( Michael Everson, Rick McGowan, Ken Whistler, V.S. Umamaheswaran) maintain the list of scripts that are candidates or potential candidates for encoding and their tentative code block assignments on the Unicode Roadmap page of the Unicode Consortium website. For some scripts on the Roadmap, such as
Jurchen Jurchen may refer to: * Jurchen people, Tungusic people who inhabited the region of Manchuria until the 17th century ** Haixi Jurchens, a grouping of the Jurchens as identified by the Chinese of the Ming Dynasty ** Jianzhou Jurchens, a grouping of ...
and
Khitan small script The Khitan small script () was one of two writing systems used for the now-extinct Khitan language (the other was the Khitan large script). It was used during the 10th–12th century by the Khitan people, who had created the Liao Empire in present ...
, encoding proposals have been made and they are working their way through the approval process. For other scripts, such as
Mayan Mayan most commonly refers to: * Maya peoples, various indigenous peoples of Mesoamerica and northern Central America * Maya civilization, pre-Columbian culture of Mesoamerica and northern Central America * Mayan languages, language family spoken ...
(besides numbers) and Rongorongo, no proposal has yet been made, and they await agreement on character repertoire and other details from the user communities involved. Some modern invented scripts which have not yet been included in Unicode (e.g., Tengwar) or which do not qualify for inclusion in Unicode due to lack of real-world use (e.g., Klingon) are listed in the
ConScript Unicode Registry The ConScript Unicode Registry is a discontinued volunteer project to coordinate the assignment of code points in the Unicode Private Use Areas (PUA) for the encoding of artificial scripts including those for constructed languages. It was founded by ...
, along with unofficial but widely used
Private Use Areas In Unicode, a Private Use Area (PUA) is a range of code points that, by definition, will not be assigned characters by the Unicode Consortium. Three private use areas are defined: one in the Basic Multilingual Plane (), and one each in, and near ...
code assignments. There is also a Medieval Unicode Font Initiative focused on special Latin medieval characters. Part of these proposals have been already included into Unicode.


Script Encoding Initiative

The Script Encoding Initiative, a project run by Deborah Anderson at the University of California, Berkeley was founded in 2002 with the goal of funding proposals for scripts not yet encoded in the standard. The project has become a major source of proposed additions to the standard in recent years.


Versions

The Unicode Consortium and the
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in A ...
(ISO) have together developed a shared
repertoire A repertoire () is a list or set of dramas, operas, musical compositions or roles which a company or person is prepared to perform. Musicians often have a musical repertoire. The first known use of the word ''repertoire'' was in 1847. It is a ...
following the initial publication of ''The Unicode Standard'' in 1991; Unicode and the ISO's Universal Coded Character Set (UCS) use identical character names and code points. However, the Unicode versions do differ from their ISO equivalents in two significant ways. While the UCS is a simple character map, Unicode specifies the rules, algorithms, and properties necessary to achieve interoperability between different platforms and languages. Thus, ''The Unicode Standard'' includes more information, covering—in depth—topics such as bitwise encoding, collation and rendering. It also provides a comprehensive catalog of character properties, including those needed for supporting bidirectional text, as well as visual charts and reference data sets to aid implementers. Previously, ''The Unicode Standard'' was sold as a print volume containing the complete core specification, standard annexes, and code charts. However, Unicode 5.0, published in 2006, was the last version printed this way. Starting with version 5.2, only the core specification, published as print-on-demand paperback, may be purchased. The full text, on the other hand, is published as a free PDF on the Unicode website. A practical reason for this publication method highlights the second significant difference between the UCS and Unicode—the frequency with which updated versions are released and new characters added. ''The Unicode Standard'' has regularly released annual expanded versions, occasionally with more than one version released in a calendar year and with rare cases where the scheduled release had to be postponed. For instance, in April 2020, only a month after version 13.0 was published, the Unicode Consortium announced they had changed the intended release date for version 14.0, pushing it back six months from March 2021 to September 2021 due to the
COVID-19 pandemic The COVID-19 pandemic, also known as the coronavirus pandemic, is an ongoing global pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The novel virus was first identi ...
. The latest version of Unicode, 15.0.0, was released on 13 September 2022. Several annexes were updated including Unicode Security Mechanisms (UTS #39), and a total of 4489 new characters were encoded, including 20 new emoji characters, such as " wireless" (network) symbol and hearts in different colors such as pink, two new scripts,
CJK Unified Ideographs The Chinese, Japanese and Korean (CJK) scripts share a common background, collectively known as CJK characters. In the process called Han unification, the common (shared) characters were identified and named CJK Unified Ideographs. As of Unicode ...
extension, and multiple additions to existing blocks. Thus far, the following major and minor versions of the Unicode standard have been published. Update versions, which do not include any changes to character repertoire, are signified by the third number (e.g., "version 4.0.1") and are omitted in the table below.


Architecture and terminology


Codespace and Code Points

The Unicode Standard defines a ''codespace,'' a set of numerical values ranging from 0 through 10FFFF 16, called '' code points'' and denoted as through ("U+" followed by the code point value in hexadecimal, which is prepended with leading zeros to a minimum of four digits; ''e. g.'', for the division sign but (''not'' ) for the Egyptian hieroglyph .). Of these 216 + 220 defined code points, the code points from through , which are used to encode surrogate pairs in UTF-16, are reserved by the Unicode Standard and may not be used to encode valid characters, resulting in a net total of 216 − 211 + 220 = 1,112,064 assignable code points.


Code planes and blocks

The Unicode codespace is divided into seventeen ''planes'', numbered 0 to 16. Plane 0 is the Basic Multilingual Plane (BMP), which contains most commonly used characters. All code points in the BMP are accessed as a single code unit in UTF-16 encoding and can be encoded in one, two or three bytes in UTF-8. Code points in Planes 1 through 16 (''supplementary planes'') are accessed as surrogate pairs in UTF-16 and encoded in four bytes in UTF-8. Within each plane, characters are allocated within named '' blocks'' of related characters. Although blocks are an arbitrary size, they are always a multiple of 16 code points and often a multiple of 128 code points. Characters required for a given script may be spread out over several different blocks.


General Category property

Each code point has a single General Category property. The major categories are denoted: Letter, Mark, Number, Punctuation, Symbol, Separator and Other. Within these categories, there are subdivisions. In most cases other properties must be used to sufficiently specify the characteristics of a code point. The possible General Categories are: Code points in the range U+D800–U+DBFF (1,024 code points) are known as high-surrogate code points, and code points in the range U+DC00–U+DFFF (1,024 code points) are known as low-surrogate code points. A high-surrogate code point followed by a low-surrogate code point form a surrogate pair in UTF-16 to represent code points greater than U+FFFF. These code points otherwise cannot be used (this rule is ignored often in practice especially when not using UTF-16). A small set of code points are guaranteed never to be used for encoding characters, although applications may make use of these code points internally if they wish. There are sixty-six of these noncharacters: U+FDD0–U+FDEF and any code point ending in the value FFFE or FFFF (i.e., U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE, U+10FFFF). The set of noncharacters is stable, and no new noncharacters will ever be defined. Like surrogates, the rule that these cannot be used is often ignored, although the operation of the byte order mark (BOM) assumes that U+FFFE will never be the first code point in a text. Excluding surrogates and noncharacters leaves 1,111,998 code points available for use. Private-use code points are considered to be assigned characters, but they have no interpretation specified by the Unicode standard so any interchange of such characters requires an agreement between sender and receiver on their interpretation. There are three private-use areas in the Unicode codespace: * Private Use Area: U+E000–U+F8FF (6,400 characters), * Supplementary Private Use Area-A: U+F0000–U+FFFFD (65,534 characters), * Supplementary Private Use Area-B: U+100000–U+10FFFD (65,534 characters). Graphic characters are characters defined by Unicode to have particular semantics, and either have a visible glyph shape or represent a visible space. As of Unicode 15.0 there are 149,014 graphic characters. Format characters are characters that do not have a visible appearance, but may have an effect on the appearance or behavior of neighboring characters. For example, and may be used to change the default shaping behavior of adjacent characters (e.g., to inhibit ligatures or request ligature formation). There are 172 format characters in Unicode 15.0. Sixty-five code points (U+0000–U+001F and U+007F–U+009F) are reserved as control codes, and correspond to the
C0 and C1 control codes The C0 and C1 control code or control character sets define control codes for use in text by computer systems that use ASCII and derivatives of ASCII. The codes represent additional information about the text, such as the position of a curso ...
defined in
ISO/IEC 6429 ISO/IEC JTC 1, entitled "Information technology", is a joint technical committee (JTC) of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). Its purpose is to develop, maintain and ...
. U+0009 (Tab), U+000A (Line Feed), and U+000D (Carriage Return) are widely used in Unicode-encoded texts. In practice the C1 code points are often improperly-translated ( mojibake) as the legacy
Windows-1252 Windows-1252 or CP-1252 ( code page 1252) is a single-byte character encoding of the Latin alphabet, used by default in the legacy components of Microsoft Windows for English and many European languages including Spanish, French, and German. ...
characters used by some English and Western European texts. Graphic characters, format characters, control code characters, and private use characters are known collectively as ''assigned characters''. Reserved code points are those code points which are available for use, but are not yet assigned. As of Unicode 15.0 there are 825,279 reserved code points.


Abstract characters

The set of graphic and format characters defined by Unicode does not correspond directly to the repertoire of ''abstract characters'' that is representable under Unicode. Unicode encodes characters by associating an abstract character with a particular code point. However, not all abstract characters are encoded as a single Unicode character, and some abstract characters may be represented in Unicode by a sequence of two or more characters. For example, a Latin small letter "i" with an ogonek, a
dot above When used as a diacritic mark, the term dot is usually reserved for the '' interpunct'' ( · ), or to the glyphs "combining dot above" ( ◌̇ ) and "combining dot below" ( ◌̣ ) which may be combined with some letters of th ...
, and an acute accent, which is required in Lithuanian, is represented by the character sequence U+012F, U+0307, U+0301. Unicode maintains a list of uniquely named character sequences for abstract characters that are not directly encoded in Unicode. All graphic, format, and private use characters have a unique and immutable name by which they may be identified. This immutability has been guaranteed since Unicode version 2.0 by the Name Stability policy. In cases where the name is seriously defective and misleading, or has a serious typographical error, a formal alias may be defined, and applications are encouraged to use the formal alias in place of the official character name. For example, has the formal alias , and has the formal alias .


Ready-made versus composite characters

Unicode includes a mechanism for modifying characters that greatly extends the supported glyph repertoire. This covers the use of combining diacritical marks that may be added after the base character by the user. Multiple combining diacritics may be simultaneously applied to the same character. Unicode also contains precomposed versions of most letter/diacritic combinations in normal use. These make conversion to and from legacy encodings simpler, and allow applications to use Unicode as an internal text format without having to implement combining characters. For example, ''é'' can be represented in Unicode as U+0065 () followed by U+0301 (), but it can also be represented as the precomposed character U+00E9 (). Thus, in many cases, users have multiple ways of encoding the same character. To deal with this, Unicode provides the mechanism of canonical equivalence. An example of this arises with
Hangul The Korean alphabet, known as Hangul, . Hangul may also be written as following South Korea's standard Romanization. ( ) in South Korea and Chosŏn'gŭl in North Korea, is the modern official writing system for the Korean language. The l ...
, the Korean alphabet. Unicode provides a mechanism for composing Hangul syllables with their individual subcomponents, known as Hangul Jamo. However, it also provides 11,172 combinations of precomposed syllables made from the most common jamo. The CJK characters currently have codes only for their precomposed form. Still, most of those characters comprise simpler elements (called
radicals Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
), so in principle Unicode could have decomposed them as it did with Hangul. This would have greatly reduced the number of required code points, while allowing the display of virtually every conceivable character (which might do away with some of the problems caused by Han unification). A similar idea is used by some input methods, such as Cangjie and Wubi. However, attempts to do this for character encoding have stumbled over the fact that Chinese characters do not decompose as simply or as regularly as Hangul does. A set of
radicals Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
was provided in Unicode 3.0 (CJK radicals between U+2E80 and U+2EFF, KangXi radicals in U+2F00 to U+2FDF, and ideographic description characters from U+2FF0 to U+2FFB), but the Unicode standard (ch. 12.2 of Unicode 5.2) warns against using
ideographic description sequences The Chinese character description languages are several proposed languages to most accurately and completely describe Chinese (or CJK) characters and information such as their list of components, list of strokes (basic and complex), their order, a ...
as an alternate representation for previously encoded characters:


Ligatures

Many scripts, including
Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...
and
Devanāgarī Devanagari ( ; , , Sanskrit pronunciation: ), also called Nagari (),Kathleen Kuiper (2010), The Culture of India, New York: The Rosen Publishing Group, , page 83 is a left-to-right abugida (a type of segmental writing system), based on the ...
, have special orthographic rules that require certain combinations of letterforms to be combined into special ligature forms. The rules governing ligature formation can be quite complex, requiring special script-shaping technologies such as ACE (Arabic Calligraphic Engine by DecoType in the 1980s and used to generate all the Arabic examples in the printed editions of the Unicode Standard), which became the proof of concept for OpenType (by Adobe and Microsoft),
Graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...
(by SIL International), or AAT (by Apple). Instructions are also embedded in fonts to tell the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
how to properly output different character sequences. A simple solution to the placement of combining marks or diacritics is assigning the marks a width of zero and placing the glyph itself to the left or right of the left sidebearing (depending on the direction of the script they are intended to be used with). A mark handled this way will appear over whatever character precedes it, but will not adjust its position relative to the width or height of the base glyph; it may be visually awkward and it may overlap some glyphs. Real stacking is impossible, but can be approximated in limited cases (for example, Thai top-combining vowels and tone marks can just be at different heights to start with). Generally this approach is only effective in monospaced fonts, but may be used as a fallback rendering method when more complex methods fail.


Standardized subsets

Several subsets of Unicode are standardized: Microsoft Windows since
Windows NT 4.0 Windows NT 4.0 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It is the direct successor to Windows NT 3.51, which was released to manufacturing on July 31, 1996, and then to retail ...
supports
WGL-4 Windows Glyph List 4, or more commonly WGL4 for short, also known as the ''Pan-European character set'', is a character repertoire on Microsoft operating systems comprising 657 Unicode characters, two of them private use. Its purpose is to provid ...
with 657 characters, which is considered to support all contemporary European languages using the Latin, Greek, or Cyrillic script. Other standardized subsets of Unicode include the Multilingual European Subsets: MES-1 (Latin scripts only, 335 characters), MES-2 (Latin, Greek and Cyrillic 1062 characters) and MES-3A & MES-3B (two larger subsets, not shown here). Note that MES-2 includes every character in MES-1 and WGL-4. The standard
DIN 91379 The DIN standard DIN 91379: "Characters and defined character sequences in Unicode for the electronic processing of names and data exchange in Europe, with CD-ROM" defines a normative subset of Unicode Latin characters, sequences of base characte ...
specifies a subset of Unicode letters, special characters, and sequences of letters and diacritic signs to allow the correct representation of names and to simplify data exchange in Europe. This specification supports all official languages of
European Union The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are located primarily in Europe, Europe. The union has a total area of ...
countries as well as the official languages of Iceland, Liechtenstein, Norway, and Switzerland, and also the German minority languages. To allow the transliteration of names in other writing systems to the Latin script according to the relevant ISO standards all necessary combinations of base letters and diacritic signs are provided. Rendering software which cannot process a Unicode character appropriately often displays it as an open rectangle, or the Unicode " replacement character" (U+FFFD, �), to indicate the position of the unrecognized character. Some systems have made attempts to provide more information about such characters. Apple's Last Resort font will display a substitute glyph indicating the Unicode range of the character, and the SIL International's Unicode Fallback font will display a box showing the hexadecimal scalar value of the character.


Mapping and encodings

Several mechanisms have been specified for storing a series of code points as a series of bytes. Unicode defines two mapping methods: the Unicode Transformation Format (UTF) encodings, and the Universal Coded Character Set (UCS) encodings. An encoding maps (possibly a subset of) the range of Unicode ''code points'' to sequences of values in some fixed-size range, termed ''code units''. All UTF encodings map code points to a unique sequence of bytes. The numbers in the names of the encodings indicate the number of bits per code unit (for UTF encodings) or the number of bytes per code unit (for UCS encodings and
UTF-1 UTF-1 is a method of transforming ISO/IEC 10646/Unicode into a stream of bytes. Its design does not provide self-synchronization, which makes searching for substrings and error recovery difficult. It reuses the ASCII printing characters for mult ...
). UTF-8 and UTF-16 are the most commonly used encodings. UCS-2 is an obsolete subset of UTF-16; UCS-4 and UTF-32 are functionally equivalent. UTF encodings include: * UTF-8, uses one to four bytes for each code point, maximizes compatibility with
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
* UTF-EBCDIC, similar to UTF-8 but designed for compatibility with EBCDIC (not part of ''The Unicode Standard'') * UTF-16, uses one or two 16-bit code units per code point, cannot encode surrogates * UTF-32, uses one 32-bit code unit per code point UTF-8 uses one to four bytes per code point and, being compact for Latin scripts and ASCII-compatible, provides the ''de facto'' standard encoding for interchange of Unicode text. It is used by FreeBSD and most recent Linux distributions as a direct replacement for legacy encodings in general text handling. The UCS-2 and UTF-16 encodings specify the Unicode byte order mark (BOM) for use at the beginnings of text files, which may be used for byte-order detection (or byte endianness detection). The BOM, code point U+FEFF, has the important property of unambiguity on byte reorder, regardless of the Unicode encoding used; U+FFFE (the result of byte-swapping U+FEFF) does not equate to a legal character, and U+FEFF in places other than the beginning of text conveys the zero-width non-break space (a character with no appearance and no effect other than preventing the formation of ligatures). The same character converted to UTF-8 becomes the byte sequence EF BB BF. The Unicode Standard allows that the BOM "can serve as signature for UTF-8 encoded text where the character set is unmarked". Some software developers have adopted it for other encodings, including UTF-8, in an attempt to distinguish UTF-8 from local 8-bit code pages. However , the UTF-8 standard, recommends that byte order marks be forbidden in protocols using UTF-8, but discusses the cases where this may not be possible. In addition, the large restriction on possible patterns in UTF-8 (for instance there cannot be any lone bytes with the high bit set) means that it should be possible to distinguish UTF-8 from other character encodings without relying on the BOM. In UTF-32 and UCS-4, one 32-bit code unit serves as a fairly direct representation of any character's code point (although the endianness, which varies across different platforms, affects how the code unit manifests as a byte sequence). In the other encodings, each code point may be represented by a variable number of code units. UTF-32 is widely used as an internal representation of text in programs (as opposed to stored or transmitted text), since every Unix operating system that uses the gcc compilers to generate software uses it as the standard "
wide character A wide character is a computer character datatype that generally has a size greater than the traditional 8-bit character. The increased datatype size allows for the use of larger coded character sets. History During the 1960s, mainframe and m ...
" encoding. Some programming languages, such as Seed7, use UTF-32 as internal representation for strings and characters. Recent versions of the Python programming language (beginning with 2.2) may also be configured to use UTF-32 as the representation for Unicode strings, effectively disseminating such encoding in
high-level High-level and low-level, as technical terms, are used to classify, describe and point to specific goals of a systematic operation; and are applied in a wide range of contexts, such as, for instance, in domains as widely varied as computer scien ...
coded software.
Punycode Punycode is a representation of Unicode with the limited ASCII character subset used for Internet hostnames. Using Punycode, host names containing Unicode characters are transcoded to a subset of ASCII consisting of letters, digits, and hyphens, wh ...
, another encoding form, enables the encoding of Unicode strings into the limited character set supported by the
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
-based
Domain Name System The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned t ...
(DNS). The encoding is used as part of IDNA, which is a system enabling the use of
Internationalized Domain Names An internationalized domain name (IDN) is an Internet domain name that contains at least one label displayed in software applications, in whole or in part, in non-latin script or alphabet, such as Arabic, Bengali, Chinese (Mandarin, simplifie ...
in all scripts that are supported by Unicode. Earlier and now historical proposals include UTF-5 and UTF-6. GB18030 is another encoding form for Unicode, from the
Standardization Administration of China The Standardization Administration of China (SAC; ; abbr.: ) is the standards organization authorized by the State Council of China to exercise administrative responsibilities by undertaking unified management, supervision and overall coordina ...
. It is the official character set of the People's Republic of China (PRC). BOCU-1 and SCSU are Unicode compression schemes. The
April Fools' Day RFC April is the fourth month of the year in the Gregorian and Julian calendars. It is the first of four months to have a length of 30 days, and the second of five months to have a length of less than 31 days. April is commonly associated with ...
of 2005 specified two parody UTF encodings,
UTF-9 A Request for Comments (RFC), in the context of Internet governance, is a type of publication from the Internet Engineering Task Force (IETF) and the Internet Society (ISOC), usually describing methods, behaviors, research, or innovations appli ...
and
UTF-18 A Request for Comments (RFC), in the context of Internet governance, is a type of publication from the Internet Engineering Task Force (IETF) and the Internet Society (ISOC), usually describing methods, behaviors, research, or innovations appli ...
.


Adoption

Unicode, in the form of UTF-8, has been the most common encoding for the World Wide Web since 2008. It has near-universal adoption, and much of the non-UTF-8 content is found in other Unicode encodings, e.g. UTF-16. , UTF-8 accounts for on average 97.8% of all web pages (and 988 of the top 1,000 highest ranked web pages). Although many pages only use
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
characters to display content, UTF-8 was designed with 8-bit ASCII as a subset and almost no websites now declare their encoding to only be ASCII instead of UTF-8. Over a third of the languages tracked have 100% UTF-8 use. All internet protocols maintained by
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements an ...
, e.g. FTP, have required support for UTF-8 since the publication of in 1998, which specified that all IETF protocols "MUST be able to use the UTF-8 charset".


Operating systems

Unicode has become the dominant scheme for internal processing and storage of text. Although a great deal of text is still stored in legacy encodings, Unicode is used almost exclusively for building new information processing systems. Early adopters tended to use UCS-2 (the fixed-length two-byte obsolete precursor to UTF-16) and later moved to UTF-16 (the variable-length current standard), as this was the least disruptive way to add support for non-BMP characters. The best known such system is Windows NT (and its descendants, 2000, XP, Vista, 7, 8, 10, and 11), which uses UTF-16 as the sole internal character encoding. The Java and .NET bytecode environments, macOS, and KDE also use it for internal representation. Partial support for Unicode can be installed on Windows 9x through the
Microsoft Layer for Unicode The Microsoft Layer for Unicode (MSLU) is a software library for legacy versions of Windows, simplifying the creation of Unicode-aware programs on Windows 9x (Windows 95, Windows 98, and Windows Me). It is also known as UnicoWS (Unicode for Windows ...
. UTF-8 (originally developed for Plan 9) has become the main storage encoding on most Unix-like operating systems (though others are also used by some libraries) because it is a relatively easy replacement for traditional extended ASCII character sets. UTF-8 is also the most common Unicode encoding used in
HTML The HyperText Markup Language or HTML is the standard markup language for documents designed to be displayed in a web browser. It can be assisted by technologies such as Cascading Style Sheets (CSS) and scripting languages such as JavaS ...
documents on the World Wide Web. Multilingual text-rendering engines which use Unicode include Uniscribe and DirectWrite for Microsoft Windows,
ATSUI The Apple Type Services for Unicode Imaging (ATSUI) is the set of services for rendering Unicode-encoded text introduced in Mac OS 8.5 and carried forward into Mac OS X. It replaced the WorldScript engine for legacy encodings. Obsolescence ...
and
Core Text Core Text is a Core Foundation style API in macOS, first introduced in Mac OS X 10.4 Tiger, made public in Mac OS X 10.5 Leopard, and introduced for the iPad with iPhone SDK 3.2. Exposing a C API, it replaces the text rendering abilities of th ...
for macOS, and Pango for GTK+ and the GNOME desktop.


Input methods

Because keyboard layouts cannot have simple key combinations for all characters, several operating systems provide alternative input methods that allow access to the entire repertoire. ISO/IEC 14755, which standardises methods for entering Unicode characters from their code points, specifies several methods. There is the ''Basic method'', where a ''beginning sequence'' is followed by the hexadecimal representation of the code point and the ''ending sequence''. There is also a ''screen-selection entry method'' specified, where the characters are listed in a table in a screen, such as with a character map program. Online tools for finding the code point for a known character include Unicode Lookup by Jonathan Hedley and Shapecatcher by Benjamin Milde. In Unicode Lookup, one enters a search key (e.g. "fractions"), and a list of corresponding characters with their code points is returned. In Shapecatcher, based on
Shape context A shape or figure is a graphical representation of an object or its external boundary, outline, or external surface, as opposed to other properties such as color, texture, or material type. A plane shape or plane figure is constrained to lie o ...
, one draws the character in a box and a list of characters approximating the drawing, with their code points, is returned.


Email

MIME defines two different mechanisms for encoding non-ASCII characters in email, depending on whether the characters are in email headers (such as the "Subject:"), or in the text body of the message; in both cases, the original character set is identified as well as a transfer encoding. For email transmission of Unicode, the UTF-8 character set and the Base64 or the
Quoted-printable Quoted-Printable, or QP encoding, is a binary-to-text encoding system using printable ASCII characters (alphanumeric and the equals sign =) to transmit 8-bit data over a 7-bit data path or, generally, over a medium which is not 8-bit clean. Hist ...
transfer encoding are recommended, depending on whether much of the message consists of
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...
characters. The details of the two different mechanisms are specified in the MIME standards and generally are hidden from users of email software. The IETF has defined a framework for internationalized email using UTF-8, and has updated several protocols in accordance with that framework. The adoption of Unicode in email has been very slow. Some East Asian text is still encoded in encodings such as
ISO-2022 ISO/IEC 2022 ''Information technology—Character code structure and extension techniques'', is an ISO/IEC standard (equivalent to the ECMA standard ECMA-35, the ANSI standard ANSI X3.41 and the Japanese Industrial Standard JIS X 0202) in the f ...
, and some devices, such as mobile phones, still cannot correctly handle Unicode data. Support has been improving, however. Many major free mail providers such as
Yahoo! Mail Yahoo! Mail is an email service launched on October 8, 1997, by the American company Yahoo, Inc. The service is free for personal use, with an optional monthly fee for additional features. Business email was previously available with the Yahoo! ...
, Gmail, and
Outlook.com Outlook.com is a webmail service that is part of the Microsoft 365 product family. It offers mail, calendaring, contacts, and tasks services. Founded in 1996 by Sabeer Bhatia and Jack Smith as Hotmail, it was acquired by Microsoft in 1997 ...
support it.


Web

All W3C recommendations have used Unicode as their ''document character set'' since HTML 4.0.
Web browser A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used o ...
s have supported Unicode, especially UTF-8, for many years. There used to be display problems resulting primarily from font related issues; e.g. v6 and older of Microsoft Internet Explorer did not render many code points unless explicitly told to use a font that contains them. Although syntax rules may affect the order in which characters are allowed to appear,
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
(including XHTML) documents, by definition, comprise characters from most of the Unicode code points, with the exception of: * most of the C0 control codes, * the permanently unassigned code points D800–DFFF, * FFFE or FFFF. HTML characters manifest either directly as
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
s according to document's encoding, if the encoding supports them, or users may write them as numeric character references based on the character's Unicode code point. For example, the references Δ, Й, ק, م, ๗, あ, 叶, 葉, and 말 (or the same numeric values expressed in hexadecimal, with &#x as the prefix) should display on all browsers as Δ, Й, ק ,م, ๗, あ, 叶, 葉, and 말. When specifying URIs, for example as URLs in HTTP requests, non-ASCII characters must be percent-encoded.


Fonts

Unicode is not in principle concerned with fonts ''per se'', seeing them as implementation choices. Any given character may have many allographs, from the more common bold, italic and base letterforms to complex decorative styles. A font is "Unicode compliant" if the glyphs in the font can be accessed using code points defined in the Unicode standard. The standard does not specify a minimum number of characters that must be included in the font; some fonts have quite a small repertoire. Free and retail fonts based on Unicode are widely available, since TrueType and OpenType support Unicode (and
Web Open Font Format The Web Open Font Format (WOFF) is a font format for use in web pages. WOFF files are OpenType or TrueType fonts, with format-specific compression applied and additional XML metadata added. The two primary goals are first to distinguish font file ...
(WOFF and
WOFF2 The Web Open Font Format (WOFF) is a font format for use in web pages. WOFF files are OpenType or TrueType fonts, with format-specific compression applied and additional XML metadata added. The two primary goals are first to distinguish font file ...
) is based on those). These font formats map Unicode code points to glyphs, but OpenType and TrueType font files are restricted to 65,535 glyphs. Collection files provide a "gap mode" mechanism for overcoming this limit in a single font file. (Each font within the collection still has the 65,535 limit, however.) A TrueType Collection file would typically have a file extension of ".ttc". Thousands of fonts exist on the market, but fewer than a dozen fonts—sometimes described as "pan-Unicode" fonts—attempt to support the majority of Unicode's character repertoire. Instead, Unicode-based fonts typically focus on supporting only basic ASCII and particular scripts or sets of characters or symbols. Several reasons justify this approach: applications and documents rarely need to render characters from more than one or two writing systems; fonts tend to demand resources in computing environments; and operating systems and applications show increasing intelligence in regard to obtaining glyph information from separate font files as needed, i.e.,
font substitution Font substitution is the process of using one typeface in place of another when the intended typeface either is not available or does not contain glyphs for the required characters. Font substitution can be aided by: * classifying fonts into ge ...
. Furthermore, designing a consistent set of rendering instructions for tens of thousands of glyphs constitutes a monumental task; such a venture passes the point of diminishing returns for most typefaces.


Newlines

Unicode partially addresses the newline problem that occurs when trying to read a text file on different platforms. Unicode defines a large number of characters that conforming applications should recognize as line terminators. In terms of the newline, Unicode introduced and . This was an attempt to provide a Unicode solution to encoding paragraphs and lines semantically, potentially replacing all of the various platform solutions. In doing so, Unicode does provide a way around the historical platform dependent solutions. Nonetheless, few if any Unicode solutions have adopted these Unicode line and paragraph separators as the sole canonical line ending characters. However, a common approach to solving this issue is through newline normalization. This is achieved with the Cocoa text system in Mac OS X and also with W3C XML and HTML recommendations. In this approach every possible newline character is converted internally to a common newline (which one does not really matter since it is an internal operation just for rendering). In other words, the text system can correctly treat the character as a newline, regardless of the input's actual encoding.


Issues


Han unification

Han unification (the identification of forms in the East Asian languages which one can treat as stylistic variations of the same historical character) has become one of the most controversial aspects of Unicode, despite the presence of a majority of experts from all three regions in the Ideographic Research Group (IRG), which advises the Consortium and ISO on additions to the repertoire and on Han unification. Unicode has been criticized for failing to separately encode older and alternative forms of kanji which, critics argue, complicates the processing of ancient Japanese and uncommon Japanese names. This is often due to the fact that Unicode encodes characters rather than glyphs (the visual representations of the basic character that often vary from one language to another). Unification of glyphs leads to the perception that the languages themselves, not just the basic character representation, are being merged.The secret life of Unicode: A peek at Unicode's soft underbelly
Suzanne Topping, 1 May 2001 ''(Internet Archive)''
There have been several attempts to create alternative encodings that preserve the stylistic differences between Chinese, Japanese, and Korean characters in opposition to Unicode's policy of Han unification. An example of one is TRON (although it is not widely adopted in Japan, there are some users who need to handle historical Japanese text and favor it). Although the repertoire of fewer than 21,000 Han characters in the earliest version of Unicode was largely limited to characters in common modern usage, Unicode now includes more than 97,000 Han characters, and work is continuing to add thousands more historic and dialectal characters used in China, Japan, Korea, Taiwan, and Vietnam. Modern font technology provides a means to address the practical issue of needing to depict a unified Han character in terms of a collection of alternative glyph representations, in the form of Unicode variation sequences. For example, the Advanced Typographic tables of OpenType permit one of a number of alternative glyph representations to be selected when performing the character to glyph mapping process. In this case, information can be provided within plain text to designate which alternate character form to select.


Italic or cursive characters in Cyrillic

If the appropriate glyphs for characters in the same script differ only in the italic, Unicode has generally unified them, as can be seen in the comparison among a set of seven characters' italic glyphs as typically appearing in Russian, traditional Bulgarian, Macedonian and Serbian texts at right, meaning that the differences are displayed through smart font technology or manually changing fonts.


Mapping to legacy character sets

Unicode was designed to provide code-point-by-code-point
round-trip format conversion {{noref, date=January 2019 The term round-trip is used in document conversion particularly involving markup languages such as XML and SGML The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized ma ...
to and from any preexisting character encodings, so that text files in older character sets can be converted to Unicode and then back and get back the same file, without employing context-dependent interpretation. That has meant that inconsistent legacy architectures, such as
combining diacritics In digital typography, combining characters are characters that are intended to modify other characters. The most common combining characters in the Latin script are the combining diacritical marks (including combining accents). Unicode also ...
and precomposed characters, both exist in Unicode, giving more than one method of representing some text. This is most pronounced in the three different encoding forms for Korean
Hangul The Korean alphabet, known as Hangul, . Hangul may also be written as following South Korea's standard Romanization. ( ) in South Korea and Chosŏn'gŭl in North Korea, is the modern official writing system for the Korean language. The l ...
. Since version 3.0, any precomposed characters that can be represented by a combining sequence of already existing characters can no longer be added to the standard in order to preserve interoperability between software using different versions of Unicode. Injective mappings must be provided between characters in existing legacy character sets and characters in Unicode to facilitate conversion to Unicode and allow interoperability with legacy software. Lack of consistency in various mappings between earlier Japanese encodings such as Shift-JIS or EUC-JP and Unicode led to
round-trip format conversion {{noref, date=January 2019 The term round-trip is used in document conversion particularly involving markup languages such as XML and SGML The Standard Generalized Markup Language (SGML; ISO 8879:1986) is a standard for defining generalized ma ...
mismatches, particularly the mapping of the character JIS X 0208 '~' (1-33, WAVE DASH), heavily used in legacy database data, to either (in
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
) or (other vendors). Some Japanese computer programmers objected to Unicode because it requires them to separate the use of and , which was mapped to 0x5C in JIS X 0201, and a lot of legacy code exists with this usage. (This encoding also replaces tilde '~' 0x7E with macron '¯', now 0xAF.) The separation of these characters exists in ISO 8859-1, from long before Unicode.


Indic scripts

Indic scripts such as
Tamil Tamil may refer to: * Tamils, an ethnic group native to India and some other parts of Asia ** Sri Lankan Tamils, Tamil people native to Sri Lanka also called ilankai tamils **Tamil Malaysians, Tamil people native to Malaysia * Tamil language, na ...
and
Devanagari Devanagari ( ; , , Sanskrit pronunciation: ), also called Nagari (),Kathleen Kuiper (2010), The Culture of India, New York: The Rosen Publishing Group, , page 83 is a left-to-right abugida (a type of segmental writing system), based on the ...
are each allocated only 128 code points, matching the ISCII standard. The correct rendering of Unicode Indic text requires transforming the stored logical order characters into visual order and the forming of ligatures (aka conjuncts) out of components. Some local scholars argued in favor of assignments of Unicode code points to these ligatures, going against the practice for other writing systems, though Unicode contains some Arabic and other ligatures for backward compatibility purposes only. Encoding of any new ligatures in Unicode will not happen, in part because the set of ligatures is font-dependent, and Unicode is an encoding independent of font variations. The same kind of issue arose for the Tibetan script in 2003 when the
Standardization Administration of China The Standardization Administration of China (SAC; ; abbr.: ) is the standards organization authorized by the State Council of China to exercise administrative responsibilities by undertaking unified management, supervision and overall coordina ...
proposed encoding 956 precomposed Tibetan syllables, but these were rejected for encoding by the relevant ISO committee ( ISO/IEC JTC 1/SC 2). Thai alphabet support has been criticized for its ordering of Thai characters. The vowels เ, แ, โ, ใ, ไ that are written to the left of the preceding consonant are in visual order instead of phonetic order, unlike the Unicode representations of other Indic scripts. This complication is due to Unicode inheriting the Thai Industrial Standard 620, which worked in the same way, and was the way in which Thai had always been written on keyboards. This ordering problem complicates the Unicode collation process slightly, requiring table lookups to reorder Thai characters for collation. Even if Unicode had adopted encoding according to spoken order, it would still be problematic to collate words in dictionary order. E.g., the word "perform" starts with a consonant cluster "สด" (with an inherent vowel for the consonant "ส"), the vowel แ-, in spoken order would come after the ด, but in a dictionary, the word is collated as it is written, with the vowel following the ส.


Combining characters

Characters with diacritical marks can generally be represented either as a single precomposed character or as a decomposed sequence of a base letter plus one or more non-spacing marks. For example, ḗ (precomposed e with macron and acute above) and ḗ (e followed by the combining macron above and combining acute above) should be rendered identically, both appearing as an e with a macron and acute accent, but in practice, their appearance may vary depending upon what rendering engine and fonts are being used to display the characters. Similarly, underdots, as needed in the romanization of Indic, will often be placed incorrectly.. Unicode characters that map to precomposed glyphs can be used in many cases, thus avoiding the problem, but where no precomposed character has been encoded the problem can often be solved by using a specialist Unicode font such as
Charis SIL Charis SIL is a transitional serif typeface developed by SIL International based on Bitstream Charter, one of the first fonts designed for laser printers. The font offers four family members: roman, bold, italic, and bold italic. Its design goal ...
that uses
Graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...
, OpenType, or AAT technologies for advanced rendering features.


Anomalies

The Unicode standard has imposed rules intended to guarantee stability. Depending on the strictness of a rule, a change can be prohibited or allowed. For example, a "name" given to a code point cannot and will not change. But a "script" property is more flexible, by Unicode's own rules. In version 2.0, Unicode changed many code point "names" from version 1. At the same moment, Unicode stated that from then on, an assigned name to a code point would never change anymore. This implies that when mistakes are published, these mistakes cannot be corrected, even if they are trivial (as happened in one instance with the spelling for in a character name). In 2006 a list of anomalies in character names was first published, and, as of June 2021, there were 104 characters with identified issues, for example: * : This is a small letter. The capital is . * : Does not join graphemes. * : This is not a Yi syllable, but a Yi iteration mark. * : ''bracket'' is spelled incorrectly. (Spelling errors are resolved by using Unicode alias names.) While Unicode defines the script designator (name) to be "", in that script's character names a hyphen is added: .


Security issues

Unicode has a large number of
homoglyphs In orthography and typography, a homoglyph is one of two or more graphemes, characters, or glyphs with shapes that appear identical or very similar. The designation is also applied to sequences of characters sharing these properties. Synoglyph ...
, many of which look very similar or identical to ASCII letters. Substitution of these can make an identifier or URL that looks correct, but directs to a different location than expected, and could also be used for manipulating the output of natural language processing (NLP) systems. Mitigation requires disallowing these characters, displaying them differently, or requiring that they resolve to the same identifier; all of this is complicated due to the huge and constantly changing set of characters. A security advisory was released in 2021 from two researchers, one from the University of Cambridge and the other from the same and from the University of Edinburgh, in which they assert that the
BIDI A bidirectional text contains two text directionalities, right-to-left (RTL) and left-to-right (LTR). It generally involves text containing different types of alphabets, but may also refer to boustrophedon, which is changing text direction in e ...
codes can be used to make large sections of code do something different from what they appear to do.


See also

* Comparison of Unicode encodings *
Religious and political symbols in Unicode Unicode contains a number of characters that represent various cultural, political, and religious symbols. Most, but not all, of these symbols are in the Miscellaneous Symbols block. The majority of them are treated as graphic symbols that ...
* International Components for Unicode (ICU), now as ICU-TC a part of Unicode * List of binary codes * List of Unicode characters * List of XML and HTML character entity references * Open-source Unicode typefaces * Standards related to Unicode * Unicode symbols * Universal Coded Character Set * Lotus Multi-Byte Character Set (LMBCS), a parallel development with similar intentions


Notes


References


Further reading

* ''The Unicode Standard, Version 3.0'', The Unicode Consortium, Addison-Wesley Longman, Inc., April 2000. * ''The Unicode Standard, Version 4.0'', The Unicode Consortium, Addison-Wesley Professional, 27 August 2003. * ''The Unicode Standard, Version 5.0, Fifth Edition'', The Unicode Consortium, Addison-Wesley Professional, 27 October 2006. * Julie D. Allen. ''The Unicode Standard, Version 6.0'', The Unicode Consortium, Mountain View, 2011, ,

. * ''The Complete Manual of Typography'', James Felici, Adobe Press; 1st edition, 2002. * ''Unicode: A Primer'', Tony Graham, M&T books, 2000. . * ''Unicode Demystified: A Practical Programmer's Guide to the Encoding Standard'', Richard Gillam, Addison-Wesley Professional; 1st edition, 2002. * ''Unicode Explained'', Jukka K. Korpela, O'Reilly; 1st edition, 2006. *


External links

*
Latest Unicode Standard
* Th
Unicode Character Database

text document
listing the names, code points and properties of all Unicode characters *
Alan Wood's Unicode Resources
contains lists of word processors with Unicode capability; fonts and characters are grouped by type; characters are presented in lists, not grids.
The World’s Writing Systems
all 294 known writing systems with their Unicode status (131 not yet encoded)
Unicode BMP Fallback Font
displays the Unicode 6.1 value of any character in a document, including in the Private Use Area, rather than the glyph itself. {{Authority control Unicode, Character encoding Digital typography