Werner Meyer-Eppler in the context of "University of Cologne"

Indeterminacy is a composing approach in which some aspects of a musical work are left open to chance or to the interpreter's free choice. John Cage, a pioneer of indeterminacy, defined it as "the ability of a piece to be performed in substantially different ways".

The earliest significant use of music indeterminacy features is found in many of the compositions of American composer Charles Ives in the early 20th century. Henry Cowell adopted Ives's ideas during the 1930s, in works allowing players to arrange the fragments of music in a number of different possible sequences. Beginning in the early 1950s, the term came to refer to the (mostly American) movement which grew up around Cage. This group included the other members of the New York School. In Europe, following the introduction of the expression "aleatory music" by Werner Meyer-Eppler, the French composer Pierre Boulez was largely responsible for popularizing the term.

Karlheinz Stockhausen (German: [kaʁlˈhaɪnts ˈʃtɔkhaʊzn̩] ; 22 August 1928 – 5 December 2007) was a German composer, widely acknowledged by critics as one of the most important but also controversial composers of the 20th and early 21st centuries. He is known for his groundbreaking work in electronic music, having been called the "father of electronic music", for introducing controlled chance (aleatory techniques) into serial composition, and for musical spatialization.

Stockhausen was educated at the Hochschule für Musik Köln and the University of Cologne, later studying with Olivier Messiaen in Paris and with Werner Meyer-Eppler at the University of Bonn. As one of the leading figures of the Darmstadt School, his compositions and theories were and remain widely influential, not only on composers of art music, but also on jazz and popular music. His works, composed over a period of nearly sixty years, eschew traditional forms. In addition to electronic music – both with and without live performers – they range from miniatures for musical boxes through works for solo instruments, songs, chamber music, choral and orchestral music, to a cycle of seven full-length operas. His theoretical and other writings comprise ten large volumes. He received numerous prizes and distinctions for his compositions, recordings, and for the scores produced by his publishing company.

Aleatoric music (also aleatory music or chance music; from the Latin word alea, meaning "dice") 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), or both. The term is most often associated with procedures in which the chance element involves a relatively limited number of possibilities.

The term became known to European composers through lectures by acoustician Werner Meyer-Eppler at the Darmstadt International Summer Courses for New Music in the beginning of the 1950s. According to his definition, "a process is said to be aleatoric ... if its course is determined in general but depends on chance in detail". Through a confusion of Meyer-Eppler's German terms Aleatorik (noun) and aleatorisch (adjective), his translator created a new English word, "aleatoric" (rather than using the existing English adjective "aleatory"), which quickly became fashionable and has persisted. More recently, the variant "aleatoriality" has been introduced.

Aleatoricism (or aleatorism) is a term for musical compositions and other forms of art resulting from "actions made by chance".

The term was first used "in the context of electro-acoustics and information theory" to describe "a course of sound events that is determined in its framework and flexible in detail", by Belgian-German physicist, acoustician, and information theorist Werner Meyer-Eppler. In practical application, in compositions by Mozart and Kirnberger, for instance, the order of the measures of a musical piece were left to be determined by throwing dice, and in performances of music by Pousseur (e.g., Répons pour sept musiciens, 1960), musicians threw dice "for sheets of music and cues". However, more generally in musical contexts, the term has had varying meanings as it was applied by various composers, and so a single, clear definition for aleatory music is defied. The term was popularised by the musical composer Pierre Boulez, but also Witold Lutosławski and Franco Evangelisti.