A quarter tone 's an interval about half as wide (aurally, or logarithmically) as a semitone, which's half a whole tone. Many composers are known for having written music including quarter tones or the quarter tone scale, first proposed by 19th-century music theorist Mikha'il Mishaqah (Touma 1996, p.16), including: Pierre Boulez, Julián Carrillo, Mildred Couper, Alberto Ginastera, Gérard Grisey, Alois Hába, Charles Ives, Tristan Murail, Krzysztof Penderecki, Giacinto Scelsi, Tui S… (More on Quarter-tone)