Bach died before finishing his last great work, "The Art of Fugue." It is a compendium of contrapuntal problems and their resolutions, a mirror of an intellect at work. The score does not indicate which instruments it was written for, and its complexity and cerebral nature argue for its remaining in a purely abstract realm, as notes on paper.

It is performed only occasionally, as in last night's performance by the Juilliard String Quartet at the Library of Congress. True to the spirit of the music, the members of the Juilliard devoted themselves to the inner workings and relationships of the counterpoint, forsaking their usual propensity for the dramatic. But, in fact, there is not much in music that is more dramatic than the sudden silence in the middle of the last passage that marks Bach's death.

The Juilliard spun out a lovely performance of this demanding set, allowing lines to move with little weight or tension but with a real sense of the texture. The second of the four canons embedded in the collection, a movement for violin and viola, was particularly touching, its serenity set off by the quick sense of pursuit that characterized the canon that preceded it. And the low tessitura of the next to last contrapuntal passage seemed to set the stage for that stunning moment when the music stops altogether.