By all available market research on the matter (which is to say, none), only the early-music researcher or budding Latin scholar could stand a chance of getting ensnared by Anonymous 4.

Which is why market research belongs nowhere near anyone making decisions about artists and repertoire. To the modern ear, the pace of chord change and melodic progress in much of Anonymous 4's repertoire might be about as gripping as watching the growth of certain molds. And yet, the female a cappella quartet has sold two million recordings since forming in 1986.

Popularity has been solid enough locally to support 14 Philadelphia Chamber Music Society appearances in 16 years, including Friday night's packing of the pews at the Church of the Holy Trinity on Rittenhouse Square. Disruption looms, however. Where will all these devotees fulfill their holiday musical-spiritual needs after next season? Anonymous 4 says we will hear the last of them in 2015-16 (though it's important to note that a previous retirement did not stick).

In a program called "On Yoolis Night," English carols and motets from the 13th to 15th centuries, the trademark sound was there as ever - pure, ethereal, ascetic. There was, in fact, variety available to anyone listening for it: the opening hymn in soaring unison, the first motet woven of a surprising range of vocal colors smartly deployed. "Gabriel fram heven-king" started with a high dancerly exultation in unison before moving to divided lines. A common convention in this repertoire brings the comfort of routine when the singers pursue independent melodies, and occasionally all converge on the same note, as if returning home. Jacqueline Horner-Kwiatek was every bit as entrancing solo, in "Qui Creavit."

The greatest tedium-defying act of the evening might have been "Ave Maria salus hominum," whose repeated melodic material revolved largely around four notes. For many fans, it is just this kind of piece that turned them into true believers - this act of making something totally captivating from spare materials. To them, it is heaven and earth in little space.