In C (1964) by Terry Riley, is for any number of players of treble instruments, that is instruments in the range of middle C and above. The score consists of fifty-three melodic cells, many of which you might find in a Baroque or Classical work.
Each one has a double bar before and after it, indicating that it can be repeated as many times as the player wishes. Each player must play all fifty-three cells in order. You can’t go back to a previous one. Throughout the performance, a pianist plays the top two Cs of the piano as a rapid eighth-note pulse, which sets the tempo of the performance. The pulse remains constant; it doesn’t slow down or speed up. Each player synchronizes his playing with the pulse. There are all sorts of internal decisions you have to make about moving forward. You have to have a sense of ensemble.
In writing simple four-part harmony, there are four ways of moving a pair of voices. If they move in opposite directions that’s called contrary motion. If they move in the same direction, keeping the same distance between them, that’s parallel motion. If they move in the same direction but vary the distance between them, that’s similar motion. And if one voice stays still while the other moves in either direction up or down, that’s called oblique motion. So in [Philip Glass’s] Music in Similar Motion everybody moves in the same direction but doesn’t necessarily keep the same distance apart.
Interlocking is a structural device common to musics of many cultures, including Indonesia and West Africa. In medieval Western art music, the hocket (“hiccup”) is made by two players sharing a common rhythmic and melodic motive. It was done for the sake of breathing or to split up the rhythmic and melodic patterns among a group of players.
[Steve Reich’s] Clapping Music is composed solely on a single twelve-beat rhythmic pattern. The score has no staff lines because there are no pitches. Both clappers repeat the twelve-beat pattern twelve times simultaneously. After twelve repetitions, the first player steps ahead an eighth-note, continuing to play the same pattern. The first clapper repeats the same pattern throughout the piece. After twelve repetitions, the second clapper steps ahead again. He’s now a quarter-note ahead. Every twelve measures the second clapper steps ahead one eighth note. After twelve repetitions he’s back in unison with the first clapper.
What’s uncanny about Clapping Music is that each composite pattern has a totally different feel to it.
– Alvin Lucier, excerpts from Repetition, Music 109