Among his better known works are the "Variations for Orchestra" (1954–5); the "Double Concerto for Harpsichord and Piano with Two Chamber Orchestras" (1959–61); the "Piano Concerto" (1964–65), written as an 85th-birthday present for Stravinsky; the "Concerto for Orchestra" (1969), loosely based on a poem by Saint-John Perse; and the "Symphony of Three Orchestras" (1976). He also composed five string quartets, of which the second and third won the Pulitzer Prize for Music in 1960 and 1973 respectively. Spaced at regular intervals throughout his mature career, they are considered by some to be the most important body of work in that medium since Bartók. "" (1993–96) is his largest orchestral work, complex in structure and featuring contrasting layers of instrumental textures, from delicate wind solos to crashing brass and percussion outbursts.