Taylor Swift, Carly Simon Sang a 'You're So Vain' Duet on Red Tour [VIDEO]

Taylor Swift saluted guest Carly Simon on stage to sing the latter's 1973 hit "You're So Vain" to a stadium of concertgoers in Massachusetts last July 28.

On Saturday, dressed in red shorts and a striped top, Swift treated her fans to a duet during the Foxborough stop of her Red Tour.

"Before I bring out my special guest, I have this question that I've always, always had," Swift declared. "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?"

The whole audience was ecstatic when Simon emerged onstage. Then the pair sang the revengeful song.

"Ladies and gentleman, give it up for one of your own, Carly Simon!" Swift roared as soon as they were done.

While neither Swift nor Simon addressed Swift's original question (who it's about), the audience didn't seem to mind. The two singers seem to share the fact that they wrote songs about their lovelife.

Swift was very happy about this that she tweeted about her experience.

"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."

Swift later said that a backstage interview with herself and Simon would air Monday on "Good Morning America," calling the performance "one of the most special nights of the tour."

"Once we get out there and we have 55,000 people in a sing-along it's just going to be like karaoke night in the stadium. Every girl out there will be thinking of someone when we're singing," Swift said.