Progressive rock sought to raise rock music to the level of “serious” art. Bands introduced unusual metres, long multi-movement compositions, complex harmonies, keyboards (organ, Mellotron, synthesizers), and concept albums with a through-narrative.
The genre's classics are King Crimson (In the Court of the Crimson King), Yes, Genesis, Pink Floyd (The Dark Side of the Moon), and Emerson, Lake & Palmer. Prog revealed that rock could be large-scale, intellectual, and formally refined without losing its energy.
