They sang for the Rolling Stones, Elvis, Frank Sinatra, U2, Sting and Michael Jackson, but they stayed out of the spotlight -- until a new documentary gave them a starring role at last.
"Twenty Feet From Stardom," which opened the Sundance Film Festival Thursday, showcases the lives of a series of mostly African American back-up singers whose voices are familiar but whose faces and names are barely known.
They have sung on some of the most popular tunes of the modern era, but who knows the names of Darlene Love, Lisa Fischer, Tata Vega, Claudia Lennear or The Waters Family?
Or Merry Clayton, the female voice on the Rolling Stones' "Gimme Shelter," possibly one of the most iconic back-up vocals in rock and roll.
"Something that struck me instantly was that it's not sex, drugs, and rock and roll -- it's a different side of the music industry," director Morgan Neville told AFP, in the Utah ski resort of Park City, hosting Sundance until January 27.
"These are people who have lived their lives making peace with the fact that they are there to disappear in the spotlight," he added.
"In a way, I was making a film about addicts. These are people who are addicted to music. And it doesn't matter if they don't make a lot of money or don't get any glory -- they just care about the music."
That doesn't mean that they never dreamed of a solo career: almost all of them have released their own albums, but none has achieved the success that could make them a star in their own right.
Lisa Fischer, the main back-up singer on Rolling Stones tours since 1989, is an exception: her debut album "So Intense" was a major success, producing a Grammy award-winning 1991 single, "How Can I Ease the Pain?"
But even she elected not to go further with her solo career, telling the documentary-maker in the film that, for her, singing was not about competition, but about sharing.
Tata Vega tells him that if she had become a solo star, she would probably not be alive today; she would have died of a drug overdose at some point along the line.
Neville said the back-up singers form a community who have a sense of sharing far more than the stars whom they accompany.
"Not everybody can sing. It's a gift. And that gift, somehow, has to be shared. It is a very spiritual thing to them. They all came out of a church, they're all very religious and I do think that they see it as a calling.
"That's what they are addicted to, they're addicted to being able to lose themselves in this greater sense of togetherness. And that is a great thing," he said.
The movie also follows a younger generation of back-up singers through Judith Hill, whose velvet voice entranced the "King of Pop" himself, Michael Jackson.
She was supposed to be the main back-up singer on his doomed London mega-show "This is It," which he was rehearsing for when he died in 2009, ending her prospects of global exposure.
Hill still hopes to have a solo career, but has to accept routine back-up singer gigs, not all of them the most glamorous, to pay the bills.
The fact is that opportunities have dried up for these golden voices: the arrival of hip-hop and grunge in the 1990s, and then the recording industry's financial troubles as new technology arrived, have hit them hard.
"And the biggest artists that use back-up vocals like Adele or Florence and The Machine, those vocals are done by Adele or Florence. They're doing their own back-up vocals on the records.
"They don't use back-up singers."