Music in Similar Motion (1969). Philip Glass /sfcmp/

SF Contemporary Music Players perform Music in Similar Motion and other works on November 11th at Herbst Theatre in San Francisco. 

Philip Glass on the piece:

The real innovation in "Similar Motion" is its sense of drama. The earlier pieces were meditative, steady-state pieces that established a mood and stayed there. But "Similar Motion" starts with one voice, then adds another playing a fourth above the original line, and then another playing a fourth below the original line, and finally a last line kicks in to complete the sound.