Taylor Swift has pretty much perfected her recipe for success when it comes to writing her songs. However, long before she was writing songs about her relationships and break-ups, the legendary Carly Simon was doing it too.

This past weekend during Taylor's RED tour, Carly Simon joined Taylor on stage to perform her #1 song from 1973 "You're So Vain", a song written by Carly long before Taylor was born about an unnamed ex-boyfriend, which Carly has still kept a secret and hasn't revealed who the song is about.

Taylor says, "When I heard 'You're So Vain,' I just thought, 'That is the best song that has ever been written. That is the most direct way that anyone has ever addressed a breakup. It's amazing!"

Catch Carly Simon rarely performing this song live with Taylor Swift.