 Its music was traditionally used within the context of concept records, and its lyrical themes tended to be "imaginative" and politically oriented.Differences have been identified between art rock and progressive rock, with art rock emphasizing avant-garde or experimental influences and "novel sonic structure", while progressive rock has been characterized as putting a greater emphasis on classically trained instrumental technique, literary content, and symphonic features.Compared to progressive rock, art rock is "more challenging, noisy and unconventional" and "less classically influenced", with more of an emphasis on avant-garde music.