Semitone (music)

In Western music, a semitone or half-tone is the interval or step in pitch between adjacent notes in a musical scale.

Many scales are used in practice. For example, some schools of ancient Greek music argued that intervals should be capable of expression as ratios of integers, while others argued for equal spacing. Western classical music typically employs twelve pitches in an octave, the so-called chromatic scale. (The octave is the musical interval between two pitches, one the double of the frequency of the other.) On the other hand, Arabic-Persian music uses 22-24 pitches, commonly accepted to be spaced an interval of a quarter-tone apart.

The interval between notes in the chromatic scale is determined by the choice of tuning, with the most common method based upon the same interval between all notes in the scale, a method called equal temperament. In this approach, the interval of the semitone corresponds to a frequency ratio between any two adjacent pitches of 21/12.