In Western music theory, a semitone is one of two parts into which a whole tone is divided. Semitones are the smallest intervals commonly harmonically and melodically used in Western music, and are among the most dissonant intervals when sounded harmonically. A progression of semitones forms a chromatic scale, which informs the layout of many fixed-pitch instruments. For example, C is adjacent to D♭ on a keyboard, so the interval between them is a semitone.