Semitone

for the printing method see halftone

A semitone (also known in the USA as a half step when equal temperament is being assumed) is a musical interval. It is the smallest interval notated and played in diatonic music (and therefore in most western music), and corresponds to the interval sounded by striking two adjacent keys on a piano keyboard (one white key and its neighbouring black key, or two white keys where there is no intermediate black key), or to the interval between two notes produced when stopping adjacent frets on a guitar, for example.

The exact size of a semitone depends on the temperament being used, and the exact semitone being considered.

Chromatic and diatonic semitones

If a semitone is notated as two notes based on the same scale degree, with one of the notes being inflected by an accidental (e.g., C and C♯), then the semitone is said to be chromatic. If it is notated as two notes based on adjacent scale degrees, (e.g., C and D♭), then the semitone is said to be diatonic. A diatonic semitone can also be called a minor second, while a chromatic semitone is sometimes called an augmented unison.

The size of a semitone

In the tuning system known as equal temperament, all the semitones making up the chromatic scale are exactly one twelfth of an octave, and all diatonic intervals can be expressed as an equivalent number of semitones. For instance an octave is as large as twelve equally tempered semitones, and an equally tempered perfect fifth equals seven equally tempered semitones. Similarly, in equal temperament, a tone (also known as a whole tone or whole step) equals two semitones. In meantone systems more generally, the chromatic semitone is smaller than the diatonic semitone.

The terms tone and semitone are often used together, in abbreviated form, to express the pitch difference between the successive notes of diatonic scales. For example, the major scale can be expressed by T-T-S-T-T-T-S. where T equals one tone and S equals one semitone.

In equal temperament the ratio of the frequencies of two notes separated by a semitone is always equal to the twelfth root of 2 ([2 ^ ] or approximately 1.059463). However, in tunings other than equal temperament the octave is not divided into equal semitones, and none of the semitones may be equal to an equally tempered semitone.

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