semiton

Semiton, often written as semitone in English, is the smallest interval commonly used in Western tonal music. In the octave, a semiton divides the octave into 12 equal parts in the standard Western tuning system known as equal temperament. In that system, the frequency ratio between two adjacent semitones is the twelfth root of two, approximately 1.05946, and each semiton is 100 cents.

There are two practical kinds of semiton in diatonic theory: the diatonic semiton and the chromatic semiton. The diatonic semiton occurs between two adjacent letters within the diatonic scale (for example E to F or B to C). The chromatic semiton occurs between a note and its alteration (for example C to C#). In equal temperament, both sizes are the same, but in historical tunings they could differ.

Notation uses accidentals to move by one semiton: sharps and flats raise or lower pitches by a semiton; the natural cancels. Enharmonic spellings (such as C# and Db) sound the same in equal temperament but are written differently.

Role: Semitons are fundamental in forming scales, melodies, and chords. The pattern of semitons and whole tones defines major and minor scales; semitons also define the steps in chromatic, melodic, and harmonic progressions.

Tuning variation: In just intonation or historical temperaments, semitones may come in two sizes and are not always equal; some microtonal systems use more than 12 semitons per octave.

Etymology: The term derives from Latin/Italian roots meaning "half tone." The spelling semiton is common in several languages, while semitone is the English form.