Semitonia

Semitonia, commonly known as the semitone, is the smallest melodic interval in most Western tuning systems. It is the distance between two adjacent notes, such as C and C sharp, or E and F. In the standard 12-tone equal-tempered system, a semitone is defined as 100 cents. The semitone is a fundamental building block of scales, melodies, and harmony, and it underpins the chromatic and diatonic steps that organize Western music.

Historically, theorists distinguish two kinds of semitone: the diatonic semitone, found between certain scale degrees (for example E–F and B–C in the major scale), and the chromatic semitone, produced by altering a note with a sharp or flat (for example C–C♯). In equal temperament these two types are the same size, but in other tuning systems they may differ in size or character. The distinction is especially relevant in historical temperaments and in some ethnic musical traditions that do not follow the 12-tone system.

Semitonia plays a central role in music theory, notation, and performance practice. It describes intervallic relationships, voice-leading, and the tuning of pitches. In microtonal and alternative tunings, the semitone may be subdivided or redefined, and different systems may use additional smaller intervals or different cent values, expanding the traditional notion of the semitone beyond 12-tone equal temperament.

Etymology traces the term to Latin semitonium from Greek semitonos, meaning “half tone.” The concept is widely used across languages with equivalent terms for the half-step interval.