Progressive rock bands pushed "rock's technical and compositional boundaries" by going beyond the standard rock or popular verse-chorus-based song structures. Progressive rock developed from late 1960s psychedelic rock.
Progressive rock songs either avoid common popular music song structures of verse-chorus-bridge, or blur the formal distinctions by extending sections or inserting musical interludes, often with exaggerated dynamics to heighten contrast between sections. Classical forms are often inserted or substituted, sometimes yielding entire suites, building on the traditional medleys of earlier rock bands. Progressive rock songs also often have extended instrumental passages, marrying the classical solo tradition with the improvisational traditions of jazz and psychedelic rock. All of these tend to add length to progressive rock songs, which may last longer than twenty minutes.