"Pet Sounds" is regarded by musicologists as an early concept album that advanced the field of music production, introducing non-standard harmonies and timbres and incorporating elements of pop, jazz, exotica, classical, and the avant-garde. The album could not be replicated live and was the first time a group departed from the usual small-ensemble electric rock band format for a whole LP. Combined with its innovative music, which was perceived as a wholly self-conscious artistic statement (or "concept"), the record furthered the cultural legitimization of popular music and was influential in the development of psychedelic music and progressive/art rock. Since 2003, "Rolling Stone" has consistently ranked "Pet Sounds" second on lists of the greatest albums of all time. In 2004, it was preserved in the National Recording Registry by the Library of Congress for being "culturally, historically, or aesthetically significant." It has been certified platinum by the RIAA, indicating over one million units sold.