Throughout the history of music, complexity has come in waves, which break. It happened in the 1960s, when the impenetrable density of serialism was stripped bare by minimalism. It happened in the 15th century, when Johannes Ockeghem wrote vocal music that featured contrapuntal intricacies worthy of a mathematical puzzle. The younger generation, composers like Josquin des Prez, admired their elder but worked against his impulses, emphasizing songfulness and clarity.

The Clerks' Group, an English vocal ensemble, sang on both sides of that mighty generational shift Saturday evening at the University of Maryland's acoustically dreadful Inn & Conference Center. The Clerks have made Ockeghem's sacred music their specialty, and have recorded all of it, splendidly, on the ASV label.

There was no complete Mass or other long work on their program; instead, the six members of the Clerks gathered in a semicircle around a facsimile of a Renaissance choir book and sang the two masters, plus a short work each from contemporaries Robert Morton, Jacob Obrecht and Matthaeus Pipelare.

The culture of Ockeghem's place and time, the Franco-Flemish Renaissance, was beginning to stress cogency in vocal music: A listener should understand what's being sung, and that voices in a choir should work together--for example, one melodic line is imitated by a second and a third. For Ockeghem, voices could work against each other, overlapping, blurring. The Clerks produced this intoxicating sound in the Offertorium from Ockeghem's Requiem, where plainchant is the unifying thread, filled in with dense, often treacherous writing for the low voices.

Immediately afterward they sang Josquin's "Nymphes des bois," a glorious lament on the death of Ockeghem in 1497, with soaring lines for soprano and male alto, in a simplified, emotionally grabbing form that the ensemble's director, Edward Wickham, describes as "leading the ear from one voice to the next."