What is it, you wonder, sitting there listening to the Juilliard Quartet retune between movements -- what is it that makes these four men such a special musical organization?

There is plenty of time for such wonderment; the Stradivari in the Library of Congress are more temperamental than the men who play them. You go to these concerts expecting to hear a few notes slightly off pitch and a lot of readjustment while the instruments get used to the difference between an auditorium full of people and the glass cases which are their regular home.

And in this as in all other respects, the Juilliard fulfills expectations.

Thursday night they played a program perfectly balanced between the classic (Mozart's Quarter in C. K. 465), the romantic (Beethoven's third "Rasumovsky" Quarter, also i C) and the moderately modern (Walter Piston's dazzling Duo for Viola and Cello). They played it with a coordination that did not trammel individual freedom, with a freshness that did not contradict the music's familiarity, with an involvement always intense but never overstated.

It was, in sum, a typical Juilliard performance; the Beethoven and the Mozart, which this quartet has surely performed hundreds of times before, were born anew in a statement of the music not quite the same as any of the others they have given or will give in the future.

This balance between fluidity and stability, as finely adjusted as the constantly shifting balance among the four players, seems to me the key to that question I posed while they were putting their Stradivari back in tune. Some musicians and ensembles (including some very good one) make up their minds on a point of interpretation and stick with it year after year. And this is commendable, or at least not reprehensible: "Grazioso" is, after all, "grazioso" and ever will be.

But Thursday night, in a minute which bears that instruction, Robert Mann took Beethoven's lilting tune and on its varied repetitions suggested several slightly different ways in which it embodies gracefulness: Samuel Rhodes suggested another nuance and the whole group engaged in a musical discussion that seemed and was as spontaneous as an impromptu meeting on a street corner.