Led Zeppelin released their fourth album on 8 November 1971. In response to the treatment they had received from critics, particularly after Led Zeppelin III, the band decided to release the fourth album with no title, though it is variously referred to as Led Zeppelin IV, Untitled, IV, or, due to the four symbols appearing on the record label, as Four Symbols, Zoso or Runes. In addition to lacking a title, the original cover featured no band name, as the group wished to be anonymous and to avoid easy pigeonholing by the press. With 37 million copies sold, Led Zeppelin IV is one of the best-selling albums in history, and its massive popularity cemented Led Zeppelin's status as superstars in the 1970s. By 2006, it had sold 23 million copies in the United States alone. The track "Stairway to Heaven", never released as a single, was the most requested and most played song on American rock radio in the 1970s. The group followed up the album's release with tours of the UK, Australasia, North America, Japan, and the UK again from late 1971 through early 1973.