The average fan hears, appreciates, maybe even sings along to the latest hits of Adele or Rihanna on the radio. But Merry Clayton hears them and shakes her head.

“She needs some SISTERS backing her up in there.”

Clayton is one of the legends of back-up singing, that almost lost art of harmonizing — not digital doubling and technical tinkerin — that added layers to the great pop hits of the ’50s, ’60s, ’70s and ’80s. The Raelettes, where Clayton got her start, backed up Ray Charles. The Ikettes added singing, dancing heat to Tina Turner’s performances — on record and on stage. That’s Merry Clayton shrieking “Raper, MURDER” in counterpoint to Mick Jagger on The Rolling Stones’ “Gimme Shelter.” She’s the “top” — the highest voice backup, to Dianna Ross on “Some Day, We’ll Be Together.”

“She’s part of a whole era of pop music history that’s just gone,” says filmmaker Morgan Neville. At the behest of the late music and movie producer Gil Friesen, Neville set out to tell the stories of Clayton, Darlene Love and scores of others in a film. “Twenty Feet From Stardom” lets these ladies out of the limelight remember their glory. And it lets the lead singers who hired them — Springsteen and Sting, Jagger and Midler — sing the praises of those unknowns who might have been good enough to take the spotlight, but never quite had the chance.

“The best musical documentary of the year,” according to The Hollywood Reporter, “Twenty Feet” gives Clayton in particular a chance to shine. Neville calls her a “natural storyteller” and to a large degree built his film around her memories of being pulled out of bed, her hair in curlers, to belt out “Gimme Shelter,” of being stopped on the way to a dinner date to flesh out tunes on Carole King’s iconic “Tapestry” album.

“Me, Darlene, the Waters family, we were pros working for that lunch money,” Clayton remembers, stopping her recollections from time to time as a song she sang on pops up in the background music of the Orlando restaurant where we meet. “We were all church girls, we all got our hair and makeup done, called each other up to find out what the others were wearing, got good and dressed up, and rode to the studio together.

“We’d come in and KILL it. Maybe the producer would say ‘You’re my top’ (highest voice), you two are my middles and you’re my bottom.’ A lot of times, we’d work that out ourselves.

“We’d do a few takes, say ‘bye,’ and go off to the next assignment.”

Neville, a documentarian who has specialized in films about everyone from Hank Williams to Iggy Pop, says that every day he spent with these various women — Sheryl Crow was once a backup singer, as was Patti Austin — he learned another forgotten piece of music history. He marvels every time Clayton, 64, pops up on a song we hear on that restaurant’s sound system.

“They’re a vital part of that era in music, and we just wanted to be able to tell their stories and maybe suggest that musicians today could benefit from that sort of community of talent.”

The film lets the various lead singers who worked with these women expound on the nature of stardom, the fame that eluded even those who got their shots at solo careers. Clayton cut records, and her version of the “Shoop Shoop Song” (“It’s In His Kiss”) was the first to hit the radio, but didn’t catch on.

For her part, Clayton says she’s almost grateful that great fame never came. “Billy Preston and others I knew were dear, dear friends of mine,” she says, remembering the pianist, Beatles back-up and solo star who died after years of drug abuse. “And he’s not around. A pop star has two, maybe three big years, and then they’re kind of done. We got to work for years and years and years. We got to stick around.”

And now, Clayton and her contemporaries have a film celebrating their work.

“This is a real blessing, coming along now. I hope people learn something and appreciate the role we played in all this.”