harmonics

Pitches produced by lightly touching a vibrating string at a nodal point rather than stopping it fully, producing a pure, flute-like overtone rather than the fundamental pitch.

In Crumb's piano writing, harmonics are produced by silently depressing keys (touching the strings without damping them) while another note sounds, or by lightly touching a string at its midpoint while a partner plays the key. The resulting sound is ethereal and bell-like — a signature of the Makrokosmos sound world.