semitones - a semitone, also called a half step or a half tone, is the smallest musical interval commonly used in western tonal music, and it is considered the most dissonant when sounded harmonically. it is defined as the interval between two adjacent notes in a 12-tone scale (e.g. from c to c♯). this implies that its size is exactly or approximately equal to 100 cents, a twelfth of an octave.