aleatory music

(from Latin, alea: "dice game") Any 20th-century music, particularly that of the 1950s and '60s, the composition or performance of which incorporates elements of chance.

In aleatory music aspects such as the ordering of a piece's sections, its rhythms, and even its pitches are decided at the moment of performance. When not purely improvising, players follow lists of arbitrary rules or interpreted "graphic" notation that merely suggest the sounds. Charles Ives and Henry Cowell had used such techniques, but John Cage became the principal figure in aleatory; other aleatory composers include Earle Brown (1926–2002), Morton Feldman (1926–87), and Pierre Boulez.

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also called  chance music 

      (aleatory from Latin alea, “dice”), 20th-century music in which chance or indeterminate elements are left for the performer to realize. The term is a loose one, describing compositions with strictly demarcated areas for improvisation according to specific directions and also unstructured pieces consisting of vague directives, such as “Play for five minutes.”

      The indeterminate portion of aleatory music commonly occurs in two areas. The performers may be told to arrange the structure of the piece—e.g., by reordering its sections or by playing sections simultaneously as they wish. The musical score may also indicate points where performers are to improvise or even to include quasi-theatrical gestures. Such requirements may give rise to inventive notation, including brackets enclosing a blacked-out space, suggesting pitch area and duration of the improvisation. Among notable aleatory works are Music of Changes (1951) for piano and Concert for Piano and Orchestra (1958), by the American composer John Cage (Cage, John), and Klavierstück XI (1956; Keyboard Piece XI), by Karlheinz Stockhausen (Stockhausen, Karlheinz) of Germany.

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