ELIZABETH (Libba) Cotten, best known as an innovative folk guitarist and composer of the classic "Freight Train," celebrates her 90th birthday tomorrow night in a special concert at Baird Auditorium. The concert is four days after her actual birth date, but Cotten is used to delays: She gave up the guitar for almost 50 years before a chance encounter started her back on the road to renown.

Born in 1893 near Chapel Hill, N.C., Cotten grew up surrounded by music, much of it made on homemade instruments. She was particularly attracted to her older brother's banjo.

"That's how I learned to play, pickin' up his banjo, 'cause I didn't have one," she says. "From a little girl on up till I was 11, he'd play and sing every night when he'd come in . . . and he played the banjo pretty good. I listened to him and that's how I learned to play it."

By the time she was 7, Cotten was able to pick out popular melodies; she also picked her own name. "My mother and father she took in laundry; he worked in the mines didn't name me. They called me Babe or Sis, called me Short, that was my home name. Then when I started to school, I was maybe 7 or 8 years old, the teacher said, 'By the way, Little Sis Neville,' that's what people called me, 'do you have a name?' I said, 'Yes, Elizabeth,' so I named myself. I don't know if I'd ever heard the name, but I had to say something!"

Like many black children of that era, Cotten wasn't able to stay in school: By age 9, she was doing day work and serving as live-in help in the Chapel Hill area. At 12 she was earning $1 a month, which her mother saved and eventually used to buy her a guitar; the cost was equivalent to four months' wages.

Because she started out on banjo, Cotten developed a unique guitar style, which she described several years ago. "The first thing I'd do, I laid the guitar flat in my lap and I worked my left hand till I could play the strings backwards and forwards. And then after I got so I could do that, then I started to chord it and get the sound of a song that I know. And if it weren't but one string I'd get that. Then finally I'd add another string to that, and kept on till I could work my fingers pretty good. And that's how I started playing with two fingers. And after I started playing with two fingers for a while, I started using three. I was just trying to see what I could do. I never had any lessons, nobody to teach me anything. I just picked it up." She still plays guitar left-handed and upside down.

As she picked up rags and dance tunes, hymns and popular songs, Cotten's style became increasingly distinctive. She had an innate sense of instant composition, fleshing out melodies with finger-picking that astounded her family and other musicians. Then, around age 14, she "got religion" and joined a Baptist church whose deacon admonished her to stop singing "those worldly songs." She stopped playing soon after, and the next 50 years were filled with everything but music: Cotten married, had a child (she now has several great-great-grandchildren), worked at various jobs and moved to Washington.

Her first job here was selling dolls at the old Lansburgh's during the Christmas season. One day, a "fine-looking lady" and her two daughters came in, and the youngest child was briefly lost. Cotten found the child, and the customer, impressed with her kindness, told Cotten to call her if she ever wanted a job. The customer was composer Ruth Crawford Seeger, wife of folk music scholar Charles Seeger, and the 10-year-old child was Peggy Seeger. Cotten, who worked for the Seegers 10 years, would sometimes pick up Peggy's guitar and call up a tune from distant memory, fingers struggling in ancient patterns.

Peggy's brother, Mike (who would go on to form a seminal old-time group, the New Lost City Ramblers, and who will join Cotten at Baird) remembers coming home from school one day in 1959 and being let in on the discovery. "It was one of those rare moments. Peggy said, 'Did you know Libba could play?' or something like that. We went into the music room and she played 'In the Sweet Bye and Bye,' which is a church song. She played it first in the very four-square church manner and then, as she does sometimes, there was a brief pause and then she played it in the ragtime style. And it was incredible, a cross between a classical parlor style and blues, which is what I think makes her music so charming; it's sparse and reserved but also just a little bit loose."

From that moment, Cotten joined the folk and traditional music circuit, quickly becoming a festival favorite and recording several fine albums; she still gives many shows and guitar workshops a year. "Every time my agent sends me, I'm ready to go," she says brightly. "I'm not going to retire till I get so I can't use my singing or my guitar strings."

"Freight Train" remains her best known song--almost every guitarist has worked through it at some point--and Cotten's style, learned and popularized by Peggy Seeger in the early '50s, was a strong influence on younger singers like Joan Baez and Judy Collins during the early '60s folk revival.

"A lot of guitar picking where you play a bass chord and some kind of melody goes back to the influence of Elizabeth Cotten," says Mike Seeger.