SHE'S A LONELY woman in a dusty attic. She's an Italian nun in a 17th-century convent. She's somebody's wife or sister or daughter or mother in a nowhere town. If she were a man, she might be famous.

Who is she? She's the woman composer whose name you have never heard. She has been making music for as long as the art existed, in ancient Greece and medieval Europe. Some have compared her to Beethoven, others to Handel. Her work, through the centuries, is an undiscovered musical territory as abundant, as high in quality and as varied in style as the forgotten baroque music that the world began to rediscover in the early 1950s. And now, she has moved out of the attics, convents and provincial towns, into concert halls and recording studios. She is very busy today, and she is beginning to find an audience.

Now, there is a recording company named after a woman and emphasizing the music of women. For the last half year, a list of women composers available on records has appeared in the Schwann record and tape catalogue. The latest list has about 90 names, more than half belonging to women who are alive today. Concerts devoted wholly or largely to the work of women are beginning to attract attention in Washington and elsewhere. Women composers now have their own international organization and their own monthly newsletter, The American Women Composer's News. It was founded in April 1976 "for the express purpose of alleviating the gross inequities that women composers have experienced in all areas of the music world."

"The best of women composers lies in the future," says Ruth Lomon, a Boston composer who has been writing music for decades. "I think it will happen, an Elizabeth Mozart."

IT'S NOT that women are finally composing music; it's rather that people are finally noticing. There have been some hard times for women composers--Fanny Mendelssohn, for example, whom most reference works describe as "sister of Felix Mendelssohn." The description is accurate but dubiously relevant. Fanny Mendelssohn was the composer of four published books of Lieder, a collection of part songs and other assorted works. Fanny's great potential was stifled by many factors. One was her father, who wrote to his daughter in 1829, "Music will perhaps become Felix's profession; whilst for you it can and must only be an ornament, never the root of your being and doing."

Brother Felix expressed this sentiment in a letter to his mother, who had asked him to induce Fanny to publish her music: "I cannot persuade her to publish anything, because it is against my views and convictions . . . Fanny, as I know her, possesses neither the inclination nor calling for authorship. She is too much a woman for that, as is proper, and looks after her house and thinks neither about the public nor the musical world, unless that primary occupation is accomplished. Publishing would only disturb her in these duties, and I cannot reconcile myself to it."

Fanny Mendelssohn is not an isolated case. Take Schumann. Not Robert--Clara. Clara Schumann, as described by Baker's biographical dictionary, was "a famous pianist, wife of Robert Schumann." Baker's goes on to say that "she was a composer in her own right; wrote a piano concerto and numerous character pieces for piano; also some songs; Schumann made use of her melodies in several of his works." Unlike Clara, Robert Schumann is a household word.

Even less fortunate are the women composers not lucky enough to be the sisters or wives of famous male composers, those whose last names are as unfamiliar as their first. For example?

Isabella Leonarda. Who? Precisely. This gifted 17th-century Italian composer published over 200 works in 20 collections of her own. It's amazing that a composer of such productivity could be mentioned so briefly in so few reference books. Isabella Leonarda would have remained unknown were it not for the ingenuity of Marnie Hall, executive producer of Leonarda Records of New York. Hall, 40, is a violinist in an all-female quartet, and in 1979 she started a record company designed to focus primarily on "outstanding women composers" of the past and present.

"I wanted to choose an acceptable name that nobody else had, a meaningful name," said Hall. And Leonarda also has a name that sounds like that of a very famous man--with a feminine ending, a neat twist.

One of Leonarda's newly released albums, "Music for the Mass by Nun Composers," features some of these women, and their choral music, a totally unexplored region. In the 16th and 17th centuries, Europe witnessed an explosion of music by women, more than half of them nuns. During the 16th century it was common practice for wealthy families to marry their daughters off to the church rather than to men because the dowries expected by the church were substantially smaller than those required by husbands.

Many of these women, coming from cultured homes, had musical training, and convent life offered a creative outlet in choral and organ music. But this reverent activity soon became threatening to those who ruled the church. Popes and bishops began to feel that some of the more talented nuns were paying too much attention to music and not enough to God. In 1728 Pope Benedict XIII issued a document stating: "In such instances at Mass and at Vespers, they are accustomed to sing music of a figured type, and to introduce, besides the organ, instruments of various types, which introduce secular melodies. Because of this practice, time is taken to learn the music, and whole days are taken away from prayer, to the detriment of the spiritual life of many of the nuns . . . But the worst part of the whole thing is that those nuns who are skilled in the art of music spend much time with persons of the same city speaking about the art of music, and the techniques of the same. Their superiors have forbidden this but all exhortations to the contrary have been ignored . . ."

Historically, women have had more success in inspiring music (by men) and in teaching music (to men) than in getting their own music performed and noticed. A classic case is that of Nadia Boulanger. She was a conductor and composer, but she made history as the director of the American School of Music in Fontainebleau, where her students included Virgil Thomson and Aaron Copland.

Although Boulanger had built up a reputation as a promising organist and conductor in her youth, she met with constant obstacles. Some of her performances were ignored by the press, others mentioned briefly, written off as "insignificant." Yet some publications gave her attention, for instance, Minerva, a major Parisian women's magazine, which named her the Princess of Music after she won the second prize in the Grand Prix de Rome. Her award was considered by many an important step forward for women's rights, and some speculated that had Boulanger been a man she would have walked away with first prize.

But these temporary splashes of recognition were not enough; Boulanger finally removed herself from the public eye, finding academia a less frustrating place to pursue music. Out of the spotlight, she guided others to develop their own creativity. Composer Frances McKay studied with Boulanger when she was 21.

"She expected more," McKay recalls. "She, by believing in me, made me believe in myself. She gave me courage whenever I felt discouraged . . . She was different, she had a hurricane quality; perhaps people were shocked to find that in a woman."

American music owes Boulanger an incalculable debt. But the world will never know what music she might have composed had she been born in the 20th century rather than the 19th.

HOW DO women get out of the house and become household words? Clara Lyle Boone, a Washington composer/pianist, has found a way. Boone owns Arsis Press, a music publishing company, and has worked most of her life to bring women's music to the public. She is a soft-spoken woman who talks about her struggle in measured, careful sentences. As a young woman in the 1950s,she went to various New York publishers with her compositions.

"They laughed at me--women didn't write music then, and if they did, nobody would publish it." She pauses, then adds, "But I got asked to dinner a lot."

Boone, growing weary of dinner invitations, dreamed of starting her own music publishing company, one that would give shelter to all the unpublished compositions of women like herself. She saved the money she earned teaching music in six different states at a variety of schools. In 1957 she left New York to teach at the Washington Cathedral's Beauvoir School, where she eventually became music director. Finally, in July 1974, Arsis Press (Arsis means "upbeat," from the Greek) became a reality. But as a businesswoman operating in what she saw as a sexist field, Boone adopted a pen name for her music that would not reveal her sex: "Lyle de Bohun," from her family archives.

The climate has changed, Boone believes: "The younger ones don't know what it's like. They don't even think about changing their names."

When Boone moved to Washington there was "no one to turn to"; she didn't know any other women composers. The Arsis Press has helped to form a unity among them.

Another thing that helps is the International League of Women Composers, an organization that passes along pertinent information and enables its 200 or more members to exchange addresses. Boone says "the old-girls network" is a good way to find out about jobs and grants. Still, no performance of her work has ever been reviewed, to her knowledge.

Frances McKay, 35, a member of the Contemporary Music Forum whose compositions have been published by Arsis Press, assesses scores submitted to the Forum--up to 100 a month. She believes "gender has nothing to do with the quality or style of music, that music composed by women is no different than that of men." But the inspiration of a piece, she says, can be gender-related. After all, she argues, a man could hardly have composed "Currents," a concerto for orchestra that deals thematically with a woman's view of life from birth to death. The work was her doctoral thesis at the Peabody Conservatory of Music. In the second movement she "tried to portray the sensation of carrying a child." She wrote part of it while pregnant with her now-5-year-old twins Daniel and Katie; she called it "Quickening." Being a mother also inspired another work, "Nursery Rhymes," which deals with the way parents read nursery rhymes, how they communicate a fantasy world to their young.

Another composer published by the Arsis Press is Ruth Lomon.

"There were so few female mentors, female models, when I began," she says. "The women's movement is helping. We're learning self-assertion, and that's a new thing for women." Composing music, she points out, "is hard enough for men, and it's even harder for women. I'm a grandmother, and one of the things in bringing up a family means assuming much more of the mothering role. When my children were growing up I didn't produce that much. Then there was getting back my self-confidence, my urge to create. Women tend to get very fragmented because of all the roles we have to play." She notes that a part of composing superior music is concentration. "You need it, especially when composing the longer pieces."

Lomon gives credit to some of the new organizations for the strides women have made in composing. "Clara Boone has made women's music accessible. We didn't have anything like that when I was starting out. The support she gives is very important."

Did Lomon use her full name on her compositions? "I used to sign my work 'R. Lomon' in the '50s and '60s. Then in the '70s, and I don't even think I did it consciously, I started using the name Ruth Lomon. It was just an attempt not to be identifiable."

THE ESTABLISHMENT of Leonarda Records has not removed all obstacles for women composers. "Many people who buy records buy only the ones they know," says Marnie Hall, "and oftentimes stores carry only stock records--it's a long struggle to build up a recording company, especially one like this. I'm trying to bring in new music. We've had widespread acceptance from the schools and libraries; I'm hoping that the general public will be interested in what we're doing."

Is Leonarda making a profit? She laughs. "I've had to loan the company $25,000 out of my own pocket."

Most people who hear about it recognize the value of such a company, but now and then Hall comes up against resistance. One journalist was interested in Leonarda until he discovered what the company was really about. Then he said, "I hate all those women things. Why should anyone be recorded just because they're women?" Hall's response: "I told him it was important for every group of people to have a sense of history . . . They think I'm only doing it because they're women; I wouldn't do it if it wasn't good."

Another critic "used to make awful little comments, even though he liked the music," she adds. One quote, from a review in Fanfare magazine, reads: "Ruth Schonthal's quartet finds her in an unusually lyrical mood, while the Belgian Lucie Vallere shows total mastery (mistressery?) of the idiom of prior eras . . ." The same reviewer, John Ditsky, in another review refers to "Leonarda's feminist bias," while adding that it presents "music of genuine value which might otherwise have continued to be overlooked."

Attitudes are quite different in American Women Composers News, which offers articles each month pertinent to the woman composer's world. Topics vary from issue to issue, but the goal is always the same: to enable the woman composer to advance in her career through information on contests and recording companies . . . and to "increase the public's awareness of women's contributions to our musical culture." The magazine also prints letters from its readers, often from women relieved to have an audience at last. Says one reader, "Certainly, most of us as women have experienced subtle prejudices against any form of success in our lives. However, artistic maturity demands that we deal with these issues and then move on to the greater struggle--that of expressing our musical visions with honesty and skill."