An interval of half a semitone (24 quarter‐tones to the octave). Introduced into Western mus. in 20th cent. with rise of interest in other cultures, quarter‐tones having always been used in Eastern mus. Poses special problems of notation. Some composers who have written in quarter‐tones have built special pfs., e.g. Hans Barth and Alois Hába. Boulez, Stockhausen, and other avant‐garde serial composers have used quarter‐tones.