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".^[1] A series of short melodic fragments that can be repeated at the discretion of musicians, In C is often cited as the first minimalist composition to make a significant impact on the public consciousness.^[2]

The piece was first performed by Steve Reich, Jon Gibson, Pauline Oliveros, Stuart Dempster, Morton Subotnick and others at the San Francisco Tape Music Center.^[3][4] It received its first recorded release in 1968 on CBS Records. Subsequent performances have been recorded many times since.

In 2022, the 1968 LP recording of In C was selected by the Library of Congress for preservation in the United States National Recording Registry as being "culturally, historically, or aesthetically significant.".^[5]

In C consists of 53 short numbered musical phrases, lasting from half a beat to 32 beats, and having from one note to twenty-five. Performers are expected to play the first phrase once in unison, after which each performer may repeat the phrase or move on to the next. Each phrase may be repeated an arbitrary number of times at the discretion of each musician in the ensemble. Each musician is expected to use the same tempo, as led by "the pulse" on piano or pitched percussion (such as xylophone or marimba) but otherwise the performers have control over which phrase they play and how many times it is repeated. Performers 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 and each performance will be different from others.^[6] 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. The first musician to reach the final numbered phrase repeats it indefinitely until all other musicians reach the same phrase, at which point they all crescendo and gradually stop playing until only "the pulse" remains and then goes silent.

As detailed in some editions of the score, it is customary for one musician ("traditionally... a beautiful girl," Riley notes in the score)^[7] 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 pre-determined rhythm.^[8]

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.

The piece begins on a C major chord (patterns one through seven) with a strong emphasis on the mediant E and the entrance of the note F which begins a series of slow progressions to other chords suggesting a few subtle and ambiguous changes of key, the last pattern being an alternation between B♭ and G. Though the polyphonic interplay of the various patterns against each other and themselves at different rhythmic displacements is of primary interest, the piece may be considered heterophonic.

The first UK performance of In C was on 18 May 1968 at Royal Institute Galleries by the Music Now Ensemble directed by Cornelius Cardew as part of a series of four Music Now, Sounds of Discovery Concerts, during May 1968.^[9][10]

The piece has been recorded a number of times:

Grand Valley State University New Music Ensemble produced an album of remixed versions of In C. A discussion of the In C remixing project including music played from three of the remixed versions can be heard in Radiolab's podcast on In C from December 14, 2009.^[16] The remixers included Jad Abumrad, Mason Bates, Jack Dangers, Dennis DeSantis, R. Luke DuBois, Mikael Karlsson/Rob Stephenson, Zoë Keating, Phil Kline, Kleerup, Glenn Kotche, David Lang, Michael Lowenstern, Paul D. Miller (DJ Spooky), Nico Muhly, Todd Reynolds and Daniel Bernard Roumain (DBR).^[17]