Density 21.5 is a piece for unaccompanied solo flute. It was commissioned in 1936 by Georges Barrère for the debut of his platinum flute. The composer, Edgard Varèse, explores every area of the flute's sonic repertoire, including using percussive sounds from the flute's keys. 21.5 grams per cubic centimeter is the density of platinum, hence the composition's name.