Entrada en la wikipedia sobre la obra IN C de Terry Riley:

In C is an aleatoric musical piece composed by Terry Riley in 1964 for any number of people, although "a group of about 35 is desired if possible but smaller or larger groups will work"[1]. As its title suggests, it is in the key of C, the simplest key to perform on the piano. It is a response to the abstract academic serialist techniques used by composers in the mid-twentieth century and is often cited as the first minimalist composition.
In C consists of 53 short, numbered musical phrases; each phrase may be repeated an arbitrary number of times. Each musician has control over which phrase he or she plays: players are encouraged to play the phrases starting at different times, even if they are playing the same phrase. The performance directions state that the musical ensemble should try to stay within two to three phrases of each other. The phrases must be played in order, although some may be skipped. As detailed in some editions of the score, it is customary for one musician ("traditionally played by a beautiful girl," Riley notes) to play the note C (in octaves) in repeated eighth notes. This drone functions as a metronome and is referred to as "The Pulse".