The period after World War II produced several challenging new developments. Of these, the three most important were the expansion of serial technique, the evolution of electronic music, and the admission of chance, choice, and improvisation. Serial procedures, originally applied by Schoenberg and his disciples solely to pitch, were expanded to include strict mathematical organization of all the parameters of musical expression, such as rhythm, dynamics, timbre and duration. Similarly, the use of electronic media may demand considerable technical and mathematical knowledge on the part of the composer; utilizing the raw resources of electronically generated sound, traditional “musical” sound, or natural sound from the world about us, music is created on magnetic tape. The composer works directly with his sonic material rather than through the medium of performers and instruments.