Anonymous 4, a quartet of female singers specializing in medieval chant and other polyphony, are not a specifically Christmas group that emerges, when the days grow short, to sing ancient carols and seasonal rarities. They are a rare mix of scholar-performers who just happen to have devoted some of their recent projects--such as the program heard at the Kennedy Center Terrace Theater on Sunday afternoon--to music that has blossomed over the millennium to celebrate the December holidays.

Despite having replaced one member, they remain as true to their original mission--exploration--as always, and they still sing with the same gentle, rhythmically swaying, folksy ease that first marked them as a group to watch in the late 1980s. Their principal virtue is that they love, but never suffocate, the medieval vocal repertoire--music that is ancient, yet still vital in performance, music that still serves its fundamental purpose: to make words (usually about God) live more vibrantly through song.

With this year's tour, Anonymous 4 brought what is, effectively, a short liturgical play of their own construction about the legend of Saint Nicholas. Yes, that Saint Nicholas, the stern 4th-century bishop of Turkish origin who evolved through a long series of convenient domestications into something like Santa Claus. The group begins with Saint Nick as a lusty friend of the people, taking famine in hand and saving virgins from the bawd. The program mixes a wide range of music celebrating the saint, who became a particular favorite of mariners for his supposed ability to calm the seas--or at least pass on pleas for calm to the right authorities.

Anonymous 4's "Legends of St. Nicholas" sounds like a seamless mix of singing--sometimes fairly simple chants, sometimes group numbers. It's more complex than that. The group has assembled from various sources a mix of motets, conductus, hymns and songs, and linked them with "readings" that they have set to standard chant formula.

Some definitions: Motets formed the most interesting musical core of the evening. The motet is the most evolved musical form on the "St. Nicholas" program, a form that shows how individual creativity and elaboration, grafted onto simple chant forms, eventually yielded independent, self-sufficient works of what we would recognize today as "composed" music.

On Sunday, the Anonymous 4 sang several, each using a variety of techniques. The musical effect of this close part singing is like watching a light pulse and wane over several minutes, never quite changing its fundamental substance but in a constant state of static motion.

Conductus (there are several varieties) in this case refers to a simpler polyphonic work, usually with a text that is religious, or serious, but not a part of any official liturgy. They formed the bulk of the program. Particularly appealing on the "St. Nicholas" program was a simple "song," called "Sainte Nicholaes," written by Godric of Yorkshire, a 12th-century English mystic. Anonymous 4 uses this haunting and simple little song as a refrain to link different stories in the program. Those stories, translated in 1438 into an earthy late-middle English (it is slightly more familiar to our ears than Chaucer), have been set to simple, repetitive chant forms by the singers themselves.

Except for one or two important thoughts that are awkwardly divided by the musical phrase, the chant works beautifully, conveying the stories musically while keeping the text as clear as a spoken recitation.

"Legends of St. Nicholas" is a successful fusion of scholarship and re-creation. It was not perfectly sung on Sunday evening (there were some tentative opening notes in the polyphonic works), but then again this is functional music. As beautiful as it is without the aid of text or study, it is music meant to convey religious understanding and sentiment. Indeed, it makes little sense to speak of a "performance" when the music is not there for mere display or casual admiration.

Anonymous 4's essentially modest and straightforward singing style, however, gives the music a dual beauty: It works as liturgical drama, and it creates an atmospheric background that is pleasing in itself.

There were two ways to sit through Sunday's program: hypnotized by the pure, open intervals and flowing triple meters, or as a participant in something like a medieval religious celebration. Both were rewarding.