Semitone

A semitone, also called a half step, is the smallest melodic interval commonly used in Western musical tuning. In twelve-tone equal temperament (the standard system in much Western music), the octave is divided into 12 equal parts, and a semitone is the interval between two adjacent pitch classes. The frequency ratio of a semitone in this system is 2^(1/12), about 1.05946, corresponding to 100 cents on the standard equal-tempered scale.

Historically, some tuning systems recognize two distinct semitone sizes. In diatonic semitone (limma) and chromatic semitone (apotome) arrangements found in just intonation and Pythagorean tuning, the semitone comes in two sizes: approximately 90.23 cents (256/243) for the diatonic semitone and approximately 113.69 cents (2187/2048) for the chromatic semitone. These two sizes together span a whole tone, about 203.9 cents. In modern twelve-tone equal temperament, the two are merged into a single 100-cent semitone.

In practice, a semitone is the distance between adjacent keys on a piano (such as C to C♯) and is the basic step used in melodies and harmonies. Semitone movement is central to the use of accidentals, which alter a note by a semitone up or down.

Beyond the standard system, many musical cultures and contemporary composers explore microtonal tunings that subdivide the octave into more or fewer than 12 equal parts, in which the precise size of a semitone can differ. Nonetheless, in Western theory, the semitone remains the fundamental unit of pitch succession and chromatic modification.