The cover of Sgt. Pepper’s Lonely Hearts Club Band is one of the most famous and notoriously dense album covers of all time. Featuring the four Beatles surrounded by various figures from history and pop culture, the artwork was both an extension of the new, more artistically inclined attitudes the band showed on Revolver and indicative of an entirely new style the band were embracing.

Designed by artists Peter Blake and Jann Haworth, the cover of Sgt. Pepper’s came to represent the boundary-pushing aspects of pop art and psychedelia. Part of the fun was combing through the different likenesses and trying to find something you recognise. But when viewed closely, most people were able to spot a curious reference to one of The Beatles’ biggest competitors in the world of pop music: The Rolling Stones.

In the right hand corner of the image, a doll sits perched on a chair wearing a sweater. That sweater contains a message: “Welcome The Rolling Stones”. How did this strange shout out get to be integrated into the most famous album cover of all time, and what does it mean?

Well, the answer comes from Adam Cooper, the son of photographer Michael Cooper. The elder Cooper’s studio was used for the shoot, and Cooper himself took the iconic photo. A young Adam was present that day and had brought along some of his toys to add to the kaleidoscope of references. One was a doll of Shirley Temple (who appears twice elsewhere in the image) that had a sweater on it. That gift was originally from Mick Jagger, who received the sweater as a gift from an American fan.

The Rolling Stones eventually repaid the reference on their own psychedelic album, Their Satanic Majesties Request. The cover of that album, which is visually reminiscent of Sgt. Pepper’s contains the faces of all four Beatles hidden within the flowers scattered around the Stones. Cooper was also the photographer for Majesties and likely had a hand in including the references to both groups.

Check out the story behind the reference down below.