Before the Benedictine monks of Santo Domingo released their surprise smash album Chant, there was already Anonymous 4.

Named for a nameless medieval Englishman who wrote about vocal poliphony in the 13th century, Anonymous 4 is a female vocal quartet that has helped bring medieval sacred and secular music back into the light.

Chant, a collection of Gregorian chants, was preceded on Billboard's classical music charts by Anonymous 4's An English Ladymass and On Yoolis Night. On Yoolis Night is a collection of English carols and motets for Christmas dating from the 13th through 15th centuries.

"This was kind of an experiment," said Johanna Rose, one-fourth of the chorus. "We thought it was worth a try."

Popular acclaim raised the group to Billboard's list of top 10 classical artists for 1993. Anonymous 4 then released its third album, Love's Illusion, a collection of French motets on courtly love from the 13th century.

The quartet, composed of Rose, Susan Hellauer, Ruth Cunningham and Marsha Genensky, has its roots in early European music ranging up through the Renaissance. With strong backgrounds in vocal and instrumental performance, they put Anonymous 4 together in 1986.

Listeners may think they hear more than four voices. Rose said that's because of the polyphonic style, which layers and interweaves the singers' voices in a way that creates "a very rich sound -- full of harmonic overtones." That richness, she said, can create a choir from a quartet.

But it's the sweet, graceful, resonant sound of the soprano and alto voices that truly sets Anonymous 4 apart from other medieval albums.

While Chant exemplifies somber men giving voice to sacred Latin songs, Anonymous 4 breaks that mold by using only women. There is strong evidence, Rose said, that nuns in medieval convents often performed this type of music, and female choirs were probably as familiar to people of the time as their male counterparts.

The masculine stereotype has hurt the genre, Rose said, but it may have helped get Anonymous 4 generate such a stir in classical circles. "It's a shame that the tradition of women singing this music was broken," she said. "But it's heard now as a new sound, so I suppose that is somewhat advantageous for us."

An Anonymous 4 concert is not like other musical performances, Rose added. "This kind of music takes a little more effort to concentrate and really appreciate," she said.

Click on an album cover to check out Anonymous 4 at Amazon.com.