In C

In C is a musical piece composed by Terry Riley in 1964 for an indefinite number of performers. He suggests "a group of about 35 is desired if possible but smaller or larger groups will work". A series of short melodic fragments, In C is a response to the abstract academic serialist techniques devised by Schoenberg that dominated Western university composers for many decades and is often cited as the first minimalist composition (though La Monte Young's drone compositions preceded it by several years, In C had a greater impact on public consciousness).

In C consists of 53 short, numbered musical phrases, lasting from half a beat to 32 beats; each phrase may be repeated an arbitrary number of times. Each musician has control over which phrase they play: players are encouraged to play the phrases starting at different times, even if they are playing the same phrase. In this way, although the melodic content of each part is predetermined, In C has elements of aleatoric music to it. 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... a beautiful girl," Riley notes in the score) to play the note C in repeated eighth notes, typically on a piano or pitched-percussion instrument (e.g. marimba). This functions as a metronome and is referred to as "The Pulse". Steve Reich introduced the idea of a rhythmic pulse to Riley, who accepted it, thus radically altering the original composition by Riley which had no rhythm.

In C has no set duration; performances can last as little as fifteen minutes or as long as several hours, although Riley indicates "performances normally average between 45 minutes and an hour and a half." The number of performers may also vary between any two performances. The original recording of the piece was created by 11 musicians (although, through overdubbing, several dozen instruments were utilized), while a performance in 2006 at the Walt Disney Concert Hall featured 124 musicians.

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