Carly Simon has performed an extra, fourth verse of You're So Vain for the first time in a new BBC documentary.
The star plays the lost stanza sitting at her home piano, hesitating as she tries to make the words fit the melody.
"This is a verse that I haven't ever sung," she explains. "I wrote it a while ago on a pad, but it never made it into the song."
But the lyrics do not reveal the subject of the song, whose identity remains one of rock's biggest riddles.
"A friend of yours revealed to me / That you'd loved me all the time," sings Simon, who is now 71.
"[You] kept it secret from your wives / You believed it was no crime."
The lyrics were previously printed in Simon's 2015 memoir, Boys In The Trees, but she has never before sung the verse.
The clip comes from the latest episode of the BBC's Classic Albums series, which looks at Simon's 1972 album No Secrets.
You're So Vain was the album's first single and introduced a rockier sound for the singer-songwriter - something she initially resisted.
"I didn't really like the direction of the record," she says in the documentary, explaining that she "fought" producer Richard Perry "all the way", until she first heard You're So Vain on the radio in a New York taxi cab.
"It sounded so good over the radio and I thought, 'no matter what the other songs sound like, I don't mind because this one turned out so well.'"
The song went on to top the US charts and reached number three in the UK. It is a critical portrait of a self-absorbed man who "walked into the party like he was walking onto a yacht".
Over the years, Simon has refused to say who the song is about; although in 2015 she told People magazine that the second verse referenced actor Warren Beatty.
"It was my [book] publisher who called up and said, 'People Magazine will put you on the cover if you tell who You're So Vain is about, or just give one verse up.'" she told the BBC.
"And so I therefore decided to give a little bit away. Now, that doesn't mean that the other two verses aren't also about Warren. It just means that the second one is."
Arlyne Rothberg, Simon's manager in the 1970s, says "endless numbers of people" had asked her to reveal the secret, but "we never talked about it at all.
"The answer wasn't as important as the game."
However, she revealed, "the subject, [the person] who it was about, loved the fact. He couldn't have been more pleased."
Classic Albums: Carly Simon - No Secrets, will be broadcast on BBC Four at 22:00 on Friday, 5 May.