Indeterminate music is music which involves indeterminacy in its composition, performance, or both. This means that contrary to conventional music, where the composition is determined and controlled by the composer, and the performance is determined and controlled by the performer, in indeterminate music the composer and/or performer relinquish control of some or all parameters of the music. For example, a composer may compose a piece of music which specifies a certain rhythm but does not specify pitch, or a composer might write a piece for unspecified instrumentation. A composer might also use indeterminate processes, such as chance operations, to produce music which is performed in a determinstic manner.

Because conventional music notation is a largely deterministic system, interminate music often uses unconventional music notation. This might be in the form of textual instructions, graphic notation, or a combination of both. An example of music notated with instructions is La Monte Young's Piano Piece for Terry Riley #1:

Push the piano up to a wall and put the flat side flush against it. Then continue pushing into the wall. Push as hard as you can. If the piano goes through the wall, keep pushing in the same direction regardless of new obstacles and continue to push as hard as you can whether the piano is stopped against an obstacle or moving. The piece is over when you are too exhausted to push any longer.

In this particular piece, the action which the performer takes is determined. However, the result of his/her action is indeterminate, at least theoretically. The length of the piece is also indeterminate.

Indeterminate music presents some very interesting issues for the performer, since it is so easy to fake a performance. Because different performances of the same piece may sound completely different, integrity is of essence to the performer. The score must be approached with "discipline, devotion and disinterestedness" so that the ideas expressed in the composition still come through. In addition, indeterminate music often demands the performer to make decisions about pitch, instrumentation or structure, and navigate the score while performing. For this reason, it is usually not possible to play indeterminate music from memory, since different performances might lead you to different places in the score.

Many performers avoid this problem by making realizations of the score. This means that they use the original score to notate one possible performance in conventional notation. With this realization, they are able to play from memory, and also to avoid the possibility that they will be led by the score into unfamiliar territory.

Composers of indeterminate music: John Cage, La Monte Young, Morton Feldman, Earle Brown, Christian Wolff, Karlheinz Stockhausen, Michael Pisaro, Cornelius Cardew, David Tudor, Terry Riley, Steve Reich, James Tenney, Alvin Lucier, Sylvano Bussotti and others.

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