A semitone (also known in the USA as a half step) 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. In music theory, an interval is the difference (a ratio or logarithmic measure) in pitch between two notes and often refers to those two notes themselves (otherwise known as a dyad). ... In Music theory, the diatonic major scale (also known as the Guido scale), from the Greek diatonikos or to stretch out, is a fundamental building block of the European-influenced musical tradition. ... A grand piano A piano is a keyboard instrument, widely used in western music for solo performance, chamber music, and accompaniment, and also as a convenient aid to composing and rehearsal. ... The layout of a typical musical keyboard A musical keyboard is the set of adjacent depressible levers on a musical instrument which cause the instrument to produce sounds. ... The neck of a steel-string acoustic guitar showing the first four frets. ... A guitar is a stringed musical instrument. ...

The exact size of a semitone depends on the temperament being used. This page is about musical systems of tuning, for the musical process of tuning see tuning. ...

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. In music or music theory a scale degree is an individual note of a scale, both its pitch and its diatonic function. ... An accidental is a musical notation symbol used to raise or lower the pitch of a note. ... The chromatic scale is any musical scale that contains more than one consecutive half-step (in other words two adjacent pairs of scale degrees or members which are separated by a semitone). ... In Music theory, the diatonic major scale (also known as the Guido scale), from the Greek diatonikos or to stretch out, is a fundamental building block of the European-influenced musical tradition. ... A minor second is the smallest of three commonly occuring musical intervals that span two diatonic scale degrees; the others being the major second and the augmented second, which are larger by one and two semitones respectively. ...

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. Equal temperament is a scheme of musical tuning in which the octave is divided into a series of equal steps (equal frequency ratios). ... The chromatic scale is any musical scale that contains more than one consecutive half-step (in other words two adjacent pairs of scale degrees or members which are separated by a semitone). ... In music, an octave (sometimes abbreviated 8ve or 8va) is the interval between one musical note and another with half or double the frequency. ... The perfect fifth or diapente is one of three musical intervals that span five diatonic scale degrees; the others being the diminished fifth, which is one semitone smaller, and the augmented fifth, which is one semitone larger. ...

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 music theory, a diatonic scale (from the Greek diatonikos, to stretch out; also known as the heptatonia prima; set form 7-35) is a seven-note musical scale comprising five whole-tone and two half-tone steps, in which the half tones are maximally separated. ... In music theory, the major scale (or major mode) is one of the diatonic scales. ...

In equal temperament the ratio of the frequencies of two notes separated by a semitone is always equal to the twelfth root of 2. 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.