She has a spontaneity on the bandnstand that matches the music she sings. And the capacity auidence at Blues Alley senses and Carmen McRae may just burst into a humorous story as easily as she sings a lyric.

Her trio, led by Marshall Otwell, starts a new tune and MrRafe begins to sing. She stops, throws her head back, laughs and says. "Wait a minute!What are you playing! It ain't what I'm singing!"

The audience laughs and the trio tries again. The easily recognizable melody is Cole Porter's "Get Out of Town."

For 90 minutes McRae moves around the bandstand like a horn player, crouching and holding the microphone like a saxophone while singing. Or she'll stand erect while performing a long, intricate passage and then dramatically release the last note she intones.

With her finely chiseled face, draped by a mass of curly ringlets, she conveys a variety of expressions that have her squinching, or looking dreamy-eyed.

Like any great chanteuse, she uses a quick gesture - an outstretched hand, a blunted palm or the flick of a finger to great effect.

However, unlike many pretenders, Carmen McRae is first and foremost a jazz singer - one of the last. Her types, she says, is in danger of becoming extinct.

With Ella Fitzergarald and Sarah Vaughan, she is in the pantheon of great living jazz vocalists. All three share a marvelous talent for improvisation. But there are differences.

Fitzgerald is effortless in her delivery and at her best a master of scat singing. Vaughan's greatest strengths are her supreme vocal control and range and ability to revise a melody through almost endless ornamentation. McRae is the master of building and releasing rhythmic tension by dragging out the beat.

There are other first-rate jazz singers - Betty Carter, Anita O'Day, Jon Hendricks, Leon Thomas - and some who use jazz techniques - Lorez Alexander, Etta Jones, Shelia Jordan, Billy Eckstine, Joe Williams.

"What's going to happen after we're gone?" McRae asks. "And you've got to realize we aren't going to be around much longer. I'm on the downhill slide!"

For those who wonder what a jazz singer is, McRae's twice-a night performances through Saturday are a brilliant exposition.

Like any true jazz singer, she reshapes a song, say by Gershwin or Porter or a jazz original by Tadd Dameron, in the manner of an imporvising instrumentalist - changing melody, shifting rhythmic phrases and substituting hermonies. At this she is a master.

She takes "Baby Won't You Please Come Home," a piece dating back to the turn of the century, and modernizes it. She performs a capella, scat sings and ends it and fires off more of that typical McRaean humor - "If you can't come, send some money, send some Acapulco gold/If you can't make it, send Sidney Poitier or Billie Dee Williams - or both.'"

McRae is that rare jazz singer who does the same wigh contemporary material by writers like James Taylor, Elton John, Barry Manilow, Paul Williams or Leon Russell.

"Jazz is a feeling," she says. "You have to feel something that's indescribable. If it has to be explained, it don't mean a damn. If the performer gets it, that's it. If the audience gets it, that's it. Jazz is an interpretation. You can take the 'Star Spangled Banner' and interpret it as jazz."

That's why Carmen McRae, 55, native of Harlem, daughter of Jamaican immigrants, is a jazz singer and Esther Satterfield isn't.

"When I came along, I was listening to Charlie Parker, Dizzy Gillespie, Thelonious Monk and cats like those at Minton's," she recalls of the early '40s when modern jazz was being forged in Upper Manhattan.

"What these cats were playing was absolutely technically correct," she said. "You had to know some music to understand what they were doing. Fortunately, I had studied piano. So I wasn't going in blind.

"These people were my chief inspirations. I still don't think anybody can play like Charlie Parker. That mother (bleeper) could play his (bleep) off.

"In the meantime I was getting experience, singing in clubs, accompanying myself. Then I left New York and went to Chicago for three years and played in piano bars. I went back to New York and got a recording contract."

She was living the jazz experience, exchanging ideas with musicians, learning her art in friendly surroundings and developing her own pace without having to do commercial.

"They don't have those kind of clubs anymore," says McRae. "That's too bad. You know, there ought to be some place where people can get their act together - small clubs where singers can sharpen their instruments. But these places don't exist. Years ago people took part in jam sessions and learned. Nowadays booking agents want to know first how many records you've sold, not how well you can sing."

"It gets back to economics. You have to relate to what's in your era. Jazz is not popular any more. It's tough out there. There're young singers like Dee Dee Bridgewater who try to sing jazz and find out they're not cut out for it and that they can't make any money. So they go into pop music."

For McRae, singers like Esther Satterfield, Jean Carn or Minnie Ripperton don't even border on jazz. "They're contemporary pop singers," she says.

That's true. These singers have some out of a different musical tradition. Like pop singers of any generation, they do not improvise to a large degree and they do not try to achieve the rhythmic strengths or melodic subtleties of a Louis Armstrong or Charlie Parker. Their performance of a song is basically the same night after night.

Jazz, for Carmen McRae, is a musical spirit that goes beyond earning huge sums of money. It's been the experience of taking something from a long tradition and adding her own special contribution. To use jazz critic Whitney Balliet's phrase, it's the sound of surprise.

"I always change the way I sing a song," said McRae. "That's what jazz is all about."