Aleatoric music

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Aleatoric music (or aleatory) is music in which some element of the composition is left to chance or some primary element of a composed work's realization is left to the determination of its performer(s). The term became known to European composers through lectures by acoustician Werner Meyer-Eppler at Darmstadt Summer School in the beginning of the 1950s. According to his definition, "aleatoric processes are such processes which have been fixed in their outline but the details of which are left to chance". Chance music is preferred by some composers.

The term-—deriving from the Latin word alea, meaning "dice"-—has come to be associated most often with procedures in which the chance element involves a relatively limited number of possibilities. The French composer Pierre Boulez was largely responsible for popularizing the term, using it to describe works that give the performer certain liberties with regard to the sequencing and repetition of parts, an approach pioneered by avant-garde American composer-theorist Henry Cowell. The term was intended by Boulez to distinguish his work from pieces composed through the application of chance operations by John Cage and Cage's aesthetic of indeterminate music or indeterminacy. Cage's Music of Changes (1951) is the first piece to be conceived largely through random procedures (Randel 2002, p.17). An earlier example is Alan Hovhaness's use of what he called the "spirit murmur" (beginning with his 1944 piano concerto Lousadzak), in which instruments repeat a phrase for a certain amount of time in an uncoordinated fashion.

Among examples of aleatory music, Klavierstück XI by Karlheinz Stockhausen features a number of elements to be performed in changing sequences, certain orchestral works of Witold Lutosławski contain music where the orchestral ensemble is not precisely dictated, and in some works by Krzysztof Penderecki characteristic sequences are repeated quickly, producing a kind of oscillating sound.

An early genre of composition that could be considered a precedent for aleatoric compositions were the Musikalische Würfelspiele or Musical Dice Games, popular in the late 18th and early 19th century. (One such dice game is attributed to Wolfgang Amadeus Mozart.) These games consisted of a sequence of musical measures, for which each measure had several possible versions, and a procedure for selecting the precise sequence based on the throwing of a number of dice.

There has been considerable confusion of the terms aleatory and indeterminate / chance music. One of Cage's pieces, HPSCHD, itself composed using chance procedures, uses music from Mozart's Musikalisches Würfelspiel, referred to above, as well as original music. He also generally used coin-tossing and other procedures depending on designs involving a pre-defined number of choices to be made. Still, both the aesthetic aims as well as the number of elements controlled by chance make the two methods clearly different. Douglas Hofstadter, writing in Gödel, Escher, Bach, thus punningly characterises some of the musical compositions of John Cage by using the acronym CAGE to stand for Composition of Aleatorically Generated Elements, in contrast to a Beautiful Aperiodic Crystal of Harmony (or BACH).

Most modern software and hardware music composition tools, synthesizers, and signal processors ("effects") provide "randomization" features to foster aleatoric composition within specified parameters. In some synthesizers, signal processors, and sequencers, randomization can be applied to almost any parameter of sound synthesis, signal processing, or scoring. This technique is employed frequently in modern electronic music.

Open form is a term sometimes used for mobile or polyvalent musical forms, where the order of movements or sections is indeterminate or left up to the performer. Roman Haubenstock-Ramati composed a series of influential "mobiles" such as Interpolation (1958). However, "open form" in music is also used in the sense defined by the art historian Heinrich Wölfflin (Renaissance und Barock, 1888) to mean a work which is fundamentally incomplete, represents an unfinished activity, or points outside of itself. In this sense, a "mobile form" can be either "open" or "closed". An example of a closed mobile musical composition is Stockhausen's Momente (1962-64/69).

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Randel, Don Michael. (2002). The Harvard Concise Dictionary of Music and Musicians. ISBN 0-674-00978-9.
Wölfflin, Heinrich. (1888). Renaissance und Barock: Eine Untersuchung über Wesen und Entstehung der Barockstils in Italien. Munich: T. Ackermann. English edition: Renaissance and Baroque. Translated by Kathrin Simon, with an introduction by Peter Murray. London: Collins, 1964; Ithaca: Cornell University Press, 1967.