'You're So Vain' video duet by Taylor Swift, Carly Simon

'You're So Vain': At Taylor Swift's concert in Foxborough, Mass., Carly Simon joined Swift in a performance of Simon's biggest hit: 'You're So Vain.' Who is the song about?

Taylor Swift and Carly Simon teamed up to sing Simon's 1972 signature hit "You're So Vain" during Swift's concert at Foxborough, Mass., on Saturday.

In the intro, Swift once more raises the question: Who's the song about?

Simon has milked the mystery for decades now, offering clues in various interviews. She has said it isn't about one person but a composite of three people. She has also said that the name of subject of the song contains the letters "A," "E," and "R."

In 2003,  Simon auctioned off the secret to raise money for charity. Dick Ebersol - an NBC executive – bid $50,000 for the secret. A few weeks later, Simon gave a private performance of the song for Ebersol and nine friends. Afterward, she revealed the name to him alone, and made him sign a confidentiality agreement.

Billboard magazine put it at No. 72 on their list of the "Greatest Songs of All Time."

Below, Simon and Swift discuss their performance in front of 55,000 people at Gillette Stadium with MTV.