Semitone

In Western music theory, a semitone (also called a half step or half tone) is one of two parts into which a whole tone is divided. Semitones are the smallest intervals commonly harmonically and melodically used in Western music, and are among the most dissonant intervals when sounded harmonically. A progression of semitones forms a chromatic scale, which informs the layout of many fixed-pitch instruments. For example, C is adjacent to D♭ on a keyboard, so the interval between them is a semitone.

In musical notation, two types of semitones are distinguished. A diatonic semitone, or minor second, arises between two notes encompassing different staff positions, e.g. from C to D♭. A chromatic semitone, or augmented unison, arises between two notes at the same staff position, e.g. from D♭ to D♮. The sum of these two intervals, the interval from C to D, is a whole tone, or major second.

The modern tuning system of 12-tone equal temperament divides the octave into twelve equal semitones, each with a frequency ratio of the twelfth root of two (100 cents). In this system, a given interval can be represented by a specific number of semitones from any starting note. For example, two semitones may represent a major second, four semitones a major third, and seven semitones a perfect fifth.

In tuning systems derived from stacking perfect fifths, i.e. Pythagorean tuning and the related meantone temperaments, diatonic semitones all have the same size, as do chromatic semitones, but the two sizes are distinct. For example, they are not the same size in Pythagorean tuning, where the diatonic semitone is distinguished from the larger chromatic semitone, or in quarter-comma meantone temperament, where the diatonic semitone is larger instead.

The condition of having semitones is called hemitonia; that of having no semitones is anhemitonia. A musical scale or chord containing semitones is called hemitonic; one without semitones is anhemitonic.