Taylor Swift and Carly Simon Sing 'You're So Vain' [VIDEO]: Who Is 'Red' Beauty Singing About, Ex-Boyfriends? John Mayer? Harry Styles?

The go-to joke when taking potshots at Taylor Swift is her penchant for writing nasty songs about any number of her famous ex-boyfriends. "Dear John" for John Mayer. "Forever and Always" for Joe Jonas. "Back To December" for Taylor Lautner. And dozens more that are rumored and debated about whom she wrote them about. But there is one celebrity breakup song that takes the cake and it's not by Swift: "You're So Vain" by Carly Simon.

Simon released the single in 1972 to describe a former lover who apparently struggled with egocentrism. What listeners really wanted to know was, who was it about? Over the years, Simon hasn't dropped many hints, suggesting it was a composite of three men. But who were the three men? Rumors include Mick Jagger, Warren Beatty, David Bowie, David Geffen, David Cassidy and Cat Stevens. Simon isn't saying anything. Swift, obviously envious of Simon's ability to write a diss to an ex, invited the vocalist to join her onstage in Foxborough, Mass. She had to ask however.

"Before I bring out my special guest, I have this question that I've always, always had," Swift said the Gillette Stadium audience. "I think a lot of you have probably had this question too, always wondered what the answer is. Who is this song, 'You're So Vain' by Carly Simon, written about?"

Simon then appeared and performed the song with Swift, much to the delight of the older audience members (and maybe a few of their children). Swift shared her own delight on Twitter after the show.

"Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!!" she wrote. "Unreal. I could not possibly look up to her more."

Got to sing 'You're So Vain' with Carly Simon at Gillette Stadium tonight!!!!! Unreal. I could not possibly look up to her more.

— Taylor Swift (@taylorswift13) July 28, 2013

Swift has made her own guest appearances this year, appearing with The Rolling Stones during their Chicago tour date to sing along with the aforementioned Jagger during "As Tears Go By." What a twisted web we weave.