Cycle has several meanings in the field of music. Acoustically, it refers to one complete vibration, the base unit of Hertz being one cycle per second. Theoretically, an interval cycle is a collection of pitch classes created by a sequence of identical intervals. Individual pieces that aggregate into larger works are considered cycles, for example, the movements of a suite, symphony, sonata, or string quartet. This definition can apply to everything from settings of the Mass or a song cycle to an opera cycle. Cycle also applies to the complete performance of an individual composer's work in one genre.