quarter-tone music
Music using intervals of half a semitone (quarter tones), dividing the octave into 24 equal parts rather than the standard 12, creating a more finely graded pitch spectrum.
In Depth
Quarter-tone music divides the standard semitone in half, giving composers 24 pitches per octave rather than 12. This creates intervals that sound "between the cracks" of the piano keyboard — pitches that Western-trained ears initially perceive as out of tune but that many world traditions use naturally. Arabic maqam, Turkish makam, and some Indian ragas all employ quarter-tone (or similar microtonal) intervals as standard elements of their pitch systems.
Western composers began exploring quarter tones in the early 20th century. Charles Ives wrote for quarter-tone pianos (two pianos tuned a quarter tone apart), Alois Hába composed operas and string quartets in quarter tones, and Julian Carrillo built custom instruments for his "Sonido 13" system. Krzysztof Penderecki and György Ligeti used quarter tones for their destabilizing expressive effect in orchestral works. The Czech-born composer Miroslav Srnka and others continue to develop quarter-tone writing. Quarter-tone notation uses special accidentals: a half-sharp and half-flat, added to the standard system.
Charles Ives wrote pieces for two pianos tuned a quarter-tone apart — the pianists sit side by side, and the combined sound creates intervals that exist in no standard Western tuning system.