Aleatoric Music

Aleatoric music uses controlled randomness inside otherwise written scores. Composers indicate notes but leave the order to the performer, hand out graphic notation that has to be interpreted, or specify ranges and let the player choose within them. Pierre Boulez's 'limited aleatory' uses choices framed by tightly composed material; Witold Lutoslawski writes 'ad libitum' passages where each player loops independently and the cloud of repetitions becomes the music. It is distinct from John Cage's chance procedures (where the composer uses dice or the I Ching at the composition stage) — aleatory typically locates the indeterminacy at the performance stage.

The approach took shape in the 1950s and 60s as a reaction against the strict serialism that had dominated postwar European composition. Cage's 'Music of Changes' (1951) and 'Music for Piano' (1952-1956) are the chance precursors; Stockhausen's 'Klavierstuck XI' (1956) and Boulez's Third Piano Sonata (1957, ongoing) opened the European 'open form' line. Lutoslawski adopted aleatory after hearing Cage's 'Second Piano Concerto' on the radio in 1960 and used it for the rest of his career.

Compare two recordings of the same piece. In a strictly notated score they should be very similar; in aleatoric work whole passages can be reordered or sound completely different in density. Lutoslawski's 'Jeux venitiens' (1961) is particularly clear: when the score says ad libitum, the texture suddenly becomes a swarm of independent lines.

Stockhausen, 'Klavierstuck XI' (1956) for open form. Boulez, 'Piano Sonata No. 3' (1957) for controlled aleatory. Lutoslawski, 'Jeux venitiens' (1961) for orchestral aleatory.

The term 'aleatoric' was coined by Werner Meyer-Eppler at the Bonn acoustics institute around 1955 and picked up by Boulez. The word's Latin root 'alea' is the same one Caesar used in 'alea iacta est' — the die is cast.