After initially being influenced by Anton Webern, Crumb became interested in exploring unusual timbres. He often asks for instruments to be played in unusual ways and several of his pieces are written for electrically amplified instruments. Another of Crumb's best known works are the four books of Makrokosmos. The first two books (1972, 1973), for solo piano, make extensive use of string piano techniques; the third, known as Music for a Summer Evening (1974), is for two pianos and percussion; the fourth, Celestial Mechanics (1979), was written for piano four-hands. The title Makrokosmos alludes to Mikrokosmos, the six books of piano pieces by Béla Bartók; like Bartók's work, Makrokosmos is a series of short character pieces.