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.
Led Zeppelin's next album, Houses of the Holy, was released in March 1973. It featured further experimentation by the band, who expanded their use of synthesisers and mellotron orchestration. The predominantly orange album cover, designed by the London-based design group Hipgnosis, depicts images of nude children climbing the Giant's Causeway in Northern Ireland. Although the children are not shown from the front, the cover was controversial at the time of the album's release. As with the band's fourth album, neither their name nor the album title was printed on the sleeve.