A quarter tone , is a pitch halfway between the usual notes of a chromatic scale or an interval about half as wide (aurally, or logarithmically) as a semitone, which is half a whole tone. Many composers are known for having written music including quarter tones or the quarter-tone scale (24 equal temperament), first proposed by 19th-century music theorist Mikha'il Mishaqah, and in 1823 by the German theorist Heinrich Richter, including:  Pierre Boulez, Julián Carrillo, Mildred Couper, George Enescu, Alberto Ginastera, Gérard Grisey, Alois Hába, Ljubica Maric, Charles Ives, Tristan Murail, Krzysztof Penderecki, Giacinto Scelsi, Ammar El Sherei, Karlheinz Stockhausen, Tui St. George Tucker, Ivan Alexandrovich Wyschnegradsky, and Iannis Xenakis (see List of quarter tone pieces).