Both the six string quartets of Barto'k and the 16 of Beethoven mark the high points of that particular musical form in their respective centuries. And there can be little doubt that without the complex, quixotic precedent of Beethoven, there would have been no somewhat comparably complex, quixotic Barto'k.

The Juilliard String Quartet illustrated this relationship at the Library of Congress Thursday night by juxtaposing two works, one by each master, that are not exactly comparable -- though they do share the fact that both composers were at the peak of their powers.

The Barto'k was his concluding quartet, his sixth and final one, a somber and lengthy work that seems to face the issues of eternity by asking more questions than it answers, ending with an eloquent coda that tapers off into a pizzicato coda that hits just exactly the right mark between bleakness and conviction.

Before the Juilliard came along, the Barto'k quartets were largely unmined material (this one was first played in 1941). The Juilliard put all six on the musical map and hardly a string quartet now exists that does not play them. But still none does so finer than the Juilliard -- those attacks, those textures, those rhythms.

The Beethoven quartet performed was the one that was the big turning point in his concept of the quartet -- his great leap forward from Haydn, the Quartet No. 7 in F major, Op. 59, No. 1. It was the first of the quartets he wrote for the Russian, Count Rasoumowsky. Beforehand, no one had written a quartet on such a symphonic scale. The Juilliard played it with the kind of intensity and breadth that it deserves.

One final, and optimistic, note: First violinist Robert Mann, who had been suffering problems with his bowing arm that led him to take a lengthy break recently, was in marvelous form -- lyric, glowing and phrasing with splendor.