The idea of progressive (shortened to prog) rock came out of the psychedelic boom of the late '60s when the desire to experiment led musicians to incorporate classical elements and intellectual attitude in their music. Also referred to as art rock because of its desire to view individual tracks as pieces of a larger composition (like movements in a symphony), this oft-libeled form emphasized grandeur through technical prowess and fantastical imagery. Prog rock, in its most derisive connotation, refers to the mystical, multi-chaptered bombastic rock popularized by the keyboard-driven excesses of Emerson Lake & Palmer and the symphonic concept records of Yes. Prog rock can also be a blanket term that includes the rock opera of the Who, the jazz rock of the Soft Machine and Henry Cow, as well as the cosmic abstractions of Popul Vuh and Gong. It continues to exert a heavy influence upon newer electronic and experimental artists, while heirs to art rock's crusty throne, such as Ozric Tentacles and Melting Euphoria, keep its cloaked spirit alive.