Carly Simon & Taylor Swift Carly Simon & Taylor Swift

Long before Taylor Swift started writing songs about her ex-boyfriends, there was the original breakup anthem: Carly Simon's "You're So Vain."

On Saturday, Simon joined Swift onstage in Foxborough, Mass., to duet on her 1972 hit.

"Every girl out there will be thinking of someone when we're singing," Swift said backstage beforehand. "When I heard 'You're So Vain,' I just thought, That's the best song that's ever been written. That is the most direct way anyone has ever addressed a breakup. It's amazing."

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While Simon agreed to share the song's inspiration with Swift, she made the pop star promise to keep it a secret.