"Revolution 9" was not the first venture by the Beatles into experimental recordings. Following Paul McCartney's lead, the group had introduced avant-garde styling in their 1966 song "Tomorrow Never Knows" and, in January 1967, they recorded an unreleased piece called "Carnival of Light". McCartney said the work was inspired by composers Stockhausen and John Cage. Stockhausen was also a favourite of Lennon, and was one of the people included on the "Sgt. Pepper" album cover. Music critic Ian MacDonald wrote that "Revolution 9" may have been influenced by Stockhausen's "Hymnen" in particular.