Semitone
In Western music, a semitone (also called a half step or half tone) is one of two parts, equal or unequal, into which a whole tone is divided. A chromatic scale divides the octave into twelve semitones, represented on a keyboard as twelve consecutive keys. For example, C is adjacent to D♭; the interval between them is a semitone. Semitones are among the most dissonant intervals when sounded harmonically.
A diatonic semitone, or minor second, arises between two notes encompassing different staff positions, e.g. from C to D♭. A chromatic semitone, or augmented unison, arises between two notes at the same staff position, e.g. from C to C♯.
The modern tuning system of 12-tone equal temperament divides the octave into twelve equal semitones, each with a frequency ratio of the twelfth root of two (100 cents). In this system, a given musical interval can be represented by a specific number of semitones from any starting note. For example, two semitones may represent a major second, four semitones a major third, and seven semitones a perfect fifth .
In tuning systems derived from stacking perfect fifths, i.e. Pythagorean tuning and the related meantone temperaments, diatonic and chromatic semitones have uniform sizes, but are distinct. For example, they are not the same size in Pythagorean tuning, where the diatonic semitone is distinguished from the larger chromatic semitone, or in quarter-comma meantone temperament, where the diatonic semitone is larger instead.
The condition of having semitones is called hemitonia; that of having no semitones is anhemitonia. A musical scale or chord containing semitones is called hemitonic; one without semitones is anhemitonic.