 Similarities are that they both describe a mostly British attempt to elevate rock music to new levels of artistic credibility, and became the instrumental analog to concept albums and rock operas, which were typically more vocal oriented.Art rock can also refer to either classically driven rock, or to a progressive rock-folk fusion.Bruce Eder's essay "The Early History of Art-Rock/Prog Rock" states that "'progressive rock,' also sometimes known as 'art rock,' or 'classical rock'" is music in which the "bands [are] playing suites, not songs; borrowing riffs from Bach, Beethoven, and Wagner instead of Chuck Berry and Bo Diddley; and using language closer to William Blake or T. S. Eliot than to Carl Perkins or Willie Dixon."