In C

In C is a musical piece composed by Terry Riley in 1964. 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.

The piece consists of 53 short, numbered musical phrases; each phrase may be repeated an arbitrary number of times. Each musician has control over which phrase she plays: players are encouraged to play the phrases starting at different times, even if they are playing the same phrase. The rules state that the musical ensemble should try to stay within 5 or 7 phrases of each other. The phrases must be played in order, although some may be skipped.

In the background, for the duration of the piece, one person plays repeated eighth notes, the note C in octaves.

The piece usually lasts about an hour.