Progressive rock (also referred to as prog rock or prog) is a subgenre of rock music that developed in the late 1960s and early 1970s as part of a "mostly British attempt to elevate rock music to new levels of artistic credibility." John Covach, in Contemporary Music Review, says that many thought it would not just "succeed the pop of the 1960s as much as take its rightful place beside the modern classical music of Stravinsky and Bartók." Progressive rock bands pushed "rock's technical and compositional boundaries" by going beyond the standard rock or popular verse-chorus-based song structures. The Oxford Companion to Music states that progressive rock bands "...explored extended musical structures which involved intricate instrumental patterns and textures and often esoteric subject matter." Additionally, the arrangements often incorporated elements drawn from classical, jazz, and later world music. Instrumentals were common, while songs with lyrics were sometimes conceptual, abstract, or based in fantasy. Progressive rock bands sometimes used "concept albums that made unified statements, usually telling an epic story or tackling a grand overarching theme." Progressive rock developed from late 1960s psychedelic rock, as part of a wide-ranging tendency in rock music of this era to draw inspiration from ever more diverse influences. The term was initially applied to the music of bands such as Pink Floyd, King Crimson, Yes, Genesis, Jethro Tull, and Emerson, Lake & Palmer, reaching its peak of popularity in the mid 1970s.