A work "for two performers" by Steve Reich, composed in 1971 (published 1972). It consists of nothing but clapping. One performer has only one bar of music, and repeats this constantly. The other starts with the same bar, synchronised with the first performer, then moves to variations and rearrangements.

The overall effect, said to be especially good in stereo under headphones, is a building-up of both complex and simple features, depending I suppose on how in phase they are at any time. They seem to slow down and speed up, though neither is actually happening. Repetitious, but not monotonous, and strangely tuneful.

The jump from one pattern to another replaced a style he had previously been using in works, of gradually getting out of phase. With CLapping Music the move to a new pattern is abrupt, though the new pattern is a phasing of the old one.

The score in its entirety, a single page, can be seen at:
On this Reich has written an explanation of what he intends it to be:

The performance begins and ends with both performers in unison at bar (1). The number of repeats of each bar should be fixed at twelve repeats per bar. Since the first performer's part does not change, it is up to the second performer to move from one bar to the next. The second performer should try to keep his or her downbeat where it is written, ie; on the first beat of each measure (not on the first beat of the group of three claps), so that his downbeat always falls on a new beat of his or her unchanging pattern.

The choice of a particular clapping sound, ie; with cupped or flat hands is left up to the performers. Whichever timbre is chosen, both performers should try to get the same one so that their two parts will blend to produce one overall resulting pattern.

Amazon site reviews of several albums featuring Clapping Music include samples. There is a 60-second excerpt of it performed by Reich and Russ Hartenberger on the album Early Works at:
Another (30 s) is by the pianist Pierre-Laurent Aimard on the mixed album African Rhythms, where a couple of Reich pieces get a cameo between Ligeti and the Aka Pygmies: