A semitone, also called a half step or a half tone,'s the smallest musical interval commonly used in Western tonal music, and it's considered the most dissonant. The most commonly written form of this interval's the minor second, notated using two adjacent letter names (for example C and D), but the augmented unison's also used, both notes having the same letter-name, with one of the notes being inflected by an accidental (for example C and C). In twelve-tone equal temperament all semitones ar… (More on Semitone)