The development of contemporary music shows an increasing dependence on concepts that are inherently variable, and obey evolving hierarchies. That is why we have already seen the series of twelve equal notes replaced by series of sound blocks of unequal density; metre replaced by series of durations and rhythmic blocks (whether dynamic cells or a number of superimposed durations); and finally dynamics and timbre no longer content with their decorative or expressive role, but, while preserving these qualities, seeking also a functional importance, which increases their power and importance.
— Boulez, Pierre. “Alea.” Stocktakings from an Apprenticeship. 1966. New York: Oxford University Press, 1991. 26-38.
