The only example of Edgard Varèse's mature music that doesn't require electronics or substantial performing forces, Density 21.5 is one of the very few true masterpieces among the limited repertory for unaccompanied flute. The work's title refers to the specific gravity of platinum; Varèse wrote Density 21.5 as an "inaugural" piece for French flutist Georges Barrère's new instrument, which was made of this precious metal.
Density 21.5 immediately predated a period during which Varèse produced little music; it was, in fact, the last work of any consequence the composer wrote until after World War II. The composer's energies during this interval were directed primarily toward the development of his ideas regarding the spatialization of sound, which reached their culmination in the seminal electronic work Poème electronique (1957-1958).
According to the composer, Density 21.5 is based on two melodic ideas -- one modal, one atonal -- and all of the subsequent material is generated from these two themes. Despite the inherent limitations of writing for an unaccompanied melodic instrument, Varèse expertly explores new areas of space and time, utilizing registral contrasts to effect polyphonic continuity. An important work whose principles proved highly influential, Density 21.5 aptly demonstrates both the singular creative powers of its creator and the expressive and technical possibilities of the instrument.