Thursday night's concert at the National Presbyterian Church celebrated and reenacted one of the most important events in the history of Western music. At first glance, there may not seem to be much excitement in a program titled "Organum: The Birth of Polyphony," even when it is performed by Sequentia, the Cologne-based group of highly specialized singers (mostly British or American) who are among the world's leading exponents of this music. But patrons at the concert could hear music taking on new depth and power as composers learned to weave voices together.

Ultimately, this concert was a celebration of the transformation that takes place when two or more notes are sounded at the same time. A note such as C, when sung together with an E and a G, is very different from that same C sung with an F or an A. Becoming part of a chord, the note acquires a new depth and spin; a composer can choose among dozens of possible B-flats, each with a slightly different flavor, and the possibilities of complexity and subtlety take a quantum leap; music no longer has only melody and rhythm, it moves through dense thickets of harmonic relationships.

That process, without which Bach, Beethoven or Stravinsky -- or any Western music of any depth -- is unthinkable, is what Sequentia celebrated, in a series of vocal numbers that became more complex as the evening's focus advanced from the 10th to the 13th century. The singing was beautifully controlled and coordinated and executed with authentic style. The rough old harmonies gave the Latin words a special power, a strength unavailable to smoother sounds.

It was curious and refreshing to hear a program in which Leonin and Perotin, the great composers of the School of Notre Dame, were seen as the culminating point rather than the beginning. As happens so often, University Community Concerts' "Olde Musicke Series," coming into town from its usual College Park venue, put standard concert experiences in a new perspective.