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The inch (symbol: in or ) is a unit of
length Length is a measure of distance. In the International System of Quantities, length is a quantity with Dimension (physical quantity), dimension distance. In most systems of measurement a Base unit (measurement), base unit for length is chosen, ...
in the British Imperial and the United States customary
systems of measurement A system of units of measurement, also known as a system of units or system of measurement, is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defi ...
. It is equal to
yard The yard (symbol: yd) is an English units, English unit of length in both the British imperial units, imperial and US United States customary units, customary systems of measurement equalling 3 foot (unit), feet or 36 inches. Sinc ...
or of a
foot The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...
. Derived from the Roman uncia ("twelfth"), the word ''inch'' is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb. Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s the inch has been based on the
metric system The metric system is a system of measurement that standardization, standardizes a set of base units and a nomenclature for describing relatively large and small quantities via decimal-based multiplicative unit prefixes. Though the rules gover ...
and defined as exactly 25.4 mm.


Name

The English word "inch" () was an early borrowing from
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
' ("one-twelfth; Roman inch; Roman ounce"). The vowel change from Latin to Old English (which became Modern English ) is known as umlaut. The consonant change from the Latin (spelled ''c'') to English is palatalisation. Both were features of Old English phonology; see and for more information. "Inch" is cognate with " ounce" (), whose separate pronunciation and spelling reflect its reborrowing in
Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman Conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English pe ...
from Anglo-Norman ''unce'' and ''ounce''. In many other European languages, the word for "inch" is the same as or derived from the word for "thumb", as a man's thumb is about an inch wide (and this was even sometimes used to define the inch). In the
Dutch language Dutch ( ) is a West Germanic languages, West Germanic language of the Indo-European language family, spoken by about 25 million people as a first language and 5 million as a second language and is the List of languages by total number of speak ...
a term for inch is ''engelse duim'' (english thumb). Examples include ("inch") and ' ("thumb"); ("thumb"); Danish and ("inch") ' ("thumb"); (whence and ); ; ka, დუიმი, ; ; ("inch") and ' ("thumb"); ("duim"); ("thumb"); ("inch") and ' ("thumb"); and ("inch") and ''tumme'' ("thumb").


Usage


Imperial or hybrid countries

The inch is a commonly used customary unit of length in the United States, Canada, and the United Kingdom. For the United Kingdom, guidance on public sector use states that, since 1 October 1995, without time limit, the inch (along with the foot) is to be used as a primary unit for road signs and related measurements of distance (with the possible exception of clearance heights and widths) and may continue to be used as a secondary or supplementary indication following a metric measurement for other purposes.


Worldwide

Inches are used for display screens (e.g.
television Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...
s and computer monitors) worldwide. It is the official Japanese standard for electronic parts, especially display screens, and is the industry standard throughout continental Europe for display screens (
Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...
being one of few countries to supplement it with centimetres in most stores). Inches are commonly used to specify the diameter of vehicle wheel rims, and the corresponding inner diameter of tyres in tyre codes.


Technical details

The international standard symbol for inch is in (see ISO 31-1, Annex A) but traditionally the inch is denoted by a double prime, which is often approximated by a double quote symbol, and the
foot The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...
by a prime, which is often approximated by an apostrophe. For example; ' can be written as 3 2. (This is akin to how the first and
second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
"cuts" of the
hour An hour (symbol: h; also abbreviated hr) is a unit of time historically reckoned as of a day and defined contemporarily as exactly 3,600 seconds ( SI). There are 60 minutes in an hour, and 24 hours in a day. The hour was initially establis ...
are likewise indicated by prime and double prime symbols, and also the first and
second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...
cuts of the degree.) Subdivisions of an inch are typically written using dyadic fractions with odd number numerators; for example, ' would be written as and not as 2.375 nor as . However, for engineering purposes fractions are commonly given to three or four places of decimals and have been for many years.


Equivalents

international inch is equal to: * centimeters (1 inch is exactly 2.54 cm) *
millimetre 330px, Different lengths as in respect of the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 metre to 1 millimetre. The millimetre (American and British English spelling differences#-re, -er, i ...
s (1 inch is exactly 25.4 mm) * or feet (approximately) * or
yard The yard (symbol: yd) is an English units, English unit of length in both the British imperial units, imperial and US United States customary units, customary systems of measurement equalling 3 foot (unit), feet or 36 inches. Sinc ...
s (approximately) * ' tenths' *
thou The word ''thou'' () is a second-person singular pronoun in English. It is now largely archaic, having been replaced in most contexts by the word '' you'', although it remains in use in parts of Northern England and in Scots (). ''Thou' ...
or mil * points or gries * PostScript points * , , or lines * computer picas * barleycorns * US survey inches * or palms (approximately) * or hands


History

The earliest known reference to the inch in England is from the '' Laws of Æthelberht'' dating to the early 7th century, surviving in a single manuscript, the '' Textus Roffensis'' from 1120. Paragraph LXVII sets out the fine for wounds of various depths: one inch, one shilling; two inches, two shillings, etc. An Anglo-Saxon unit of length was the barleycorn. After 1066, 1 inch was equal to 3 barleycorns, which continued to be its legal definition for several centuries, with the barleycorn being the base unit. One of the earliest such definitions is that of 1324, where the legal definition of the inch was set out in a statute of Edward II of England, defining it as "three grains of
barley Barley (), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains; it was domesticated in the Fertile Crescent around 9000 BC, giving it nonshattering spikele ...
, dry and round, placed end to end, lengthwise". Similar definitions are recorded in both English and Welsh medieval law tracts. One, dating from the first half of the 10th century, is contained in the Laws of Hywel Dda which superseded those of Dyfnwal, an even earlier definition of the inch in Wales. Both definitions, as recorded in ''Ancient Laws and Institutes of Wales'' (vol i., pp. 184, 187, 189), are that "three lengths of a barleycorn is the inch". King David I of Scotland in his Assize of Weights and Measures (c. 1150) is said to have defined the Scottish inch as the width of an average man's thumb at the base of the nail, even including the requirement to calculate the average of a small, a medium, and a large man's measures. However, the oldest surviving manuscripts date from the early 14th century and appear to have been altered with the inclusion of newer material. In 1814, Charles Butler, a mathematics teacher at Cheam School, recorded the old legal definition of the inch to be "three grains of sound ripe barley being taken out the middle of the ear, well dried, and laid end to end in a row", and placed the barleycorn, not the inch, as the base unit of the English Long Measure system, from which all other units were derived. John Bouvier similarly recorded in his 1843 law dictionary that the barleycorn was the fundamental measure. Butler observed, however, that " the length of the barley-corn cannot be fixed, so the inch according to this method will be uncertain", noting that a standard inch measure was now .e. by 1843kept in the Exchequer chamber, Guildhall, and ''that'' was the legal definition of the inch. This was a point also made by George Long in his 1842 '' Penny Cyclopædia'', observing that standard measures had since surpassed the barleycorn definition of the inch, and that to recover the inch measure from its original definition, in case the standard measure were destroyed, would involve the measurement of large numbers of barleycorns and taking their average lengths. He noted that this process would not perfectly recover the standard, since it might introduce errors of anywhere between one hundredth and one tenth of an inch in the definition of a yard. Before the adoption of the international yard and pound, various definitions were in use. In the United Kingdom and most countries of the British Commonwealth, the inch was defined in terms of the Imperial Standard Yard. The United States adopted the conversion factor 1 metre = 39.37 inches by an act in 1866. In 1893, Mendenhall ordered the physical realization of the inch to be based on the international prototype metres numbers 21 and 27, which had been received from the CGPM, together with the previously adopted conversion factor. As a result of the definitions above, the U.S. inch was effectively defined as 25.4000508 mm (with a reference temperature of 68 degrees Fahrenheit) and the UK inch at 25.399977 mm (with a reference temperature of 62 degrees Fahrenheit). When Carl Edvard Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the ''de facto'' standard for manufacturers internationally, with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his. In 1930, the
British Standards Institution The British Standards Institution (BSI) is the Standards organization, national standards body of the United Kingdom. BSI produces technical standards on a wide range of products and services and also supplies standards certification services ...
adopted an inch of exactly 25.4 mm. The American Standards Association followed suit in 1933. By 1935, industry in 16 countries had adopted the "industrial inch" as it came to be known, effectively endorsing Johansson's pragmatic choice of conversion ratio. In 1946, the Commonwealth Science Congress recommended a yard of exactly 0.9144 metres for adoption throughout the British Commonwealth. This was adopted by Canada in 1951; the United States on 1 July 1959; Australia in 1961, effective 1 January 1964; and the United Kingdom in 1963, effective on 1 January 1964. The new standards gave an inch of exactly 25.4 mm, 1.7 millionths of an inch longer than the old imperial inch and 2 millionths of an inch shorter than the old US inch.


Related units


US survey inches

The United States retained the -metre definition for surveying, producing a 2 millionth part difference between standard and US survey inches.A. V. Astin & H. Arnold Karo, (1959)
''Refinement of values for the yard and the pound''
Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59-5442, Filed, 30 June 1959, 8:45 am)
This is approximately  inch per mile; 12.7 kilometres is exactly standard inches and exactly survey inches. This difference is substantial when doing calculations in State Plane Coordinate Systems with coordinate values in the hundreds of thousands or millions of feet. In 2020, the
National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...
announced that the U.S. survey foot would "be phased out" on 1 January 2023 and be superseded by the international foot (also known as the foot) equal to 0.3048 metres exactly, for all further applications. This implies that the survey inch was replaced by the international inch.


Continental inches

Before the adoption of the metric system, several European countries had customary units whose name translates into "inch". The French '' pouce'' measured roughly 27.0 mm, at least when applied to describe the calibre of artillery pieces. The Amsterdam foot (''voet'') consisted of 11 Amsterdam inches (''duim''). The Amsterdam foot is about 8% shorter than an English foot.


Scottish inch

The now obsolete Scottish inch (), of a Scottish foot, was about 1.0016 imperial inches (about ).


See also

*
English units English units were the units of measurement used in England up to 1826 (when they were replaced by Imperial units), which evolved as a combination of the Anglo-Saxons, Anglo-Saxon and Ancient Roman units of measurement, Roman systems of units. V ...
* Square inch and
Cubic inch The cubic inch (symbol in3) is a unit of volume in the Imperial units and United States customary units systems. It is the volume of a cube with each of its three dimensions (length, width, and height) being one inch long which is equivalent ...
* International yard and pound *
Anthropic units The term anthropic unit (from Greek ''wikt:άνθρωπος, άνθρωπος'', 'human') is used with different meanings in archaeology, in measurement and in social studies. In archaeology In archaeology, ''anthropic units'' are strata or de ...
* Pyramid inch * Digit and Line


Notes


References


Citations


Bibliography

* {{Authority control Customary units of measurement in the United States Imperial units Units of length Obsolete Scottish units of measurement fy:Tomme (lingtemaat)