Piano trios, regardless of the composer, tend to follow standard molds, and to come in threes. There are the inevitable three instruments: violin, cello and piano. Most of them are in three movements. And most trio programs have three works.

So for a group like the Beaux Arts Trio, which plays nothing but trios for a living, it must be a relief to break the mold a little, as they did last night at the Library of Congress. They went so far as to throw in a Dvorak Trio, the "Dumky," that has six movements, though mathematicians might argue that two times three doesn't really break the mold.

Whatever the meaning, hearing the "Dumky," which is an uninhibited Slavic romp of a work, was great fun. Instead of the normally genteel sounds of most chamber music, here were whirling dances and earthy scrapings of the violin bow.

This trio seemed to grow easily out of the Haydn trio that opened the concert. There's often a little Slavic touch in Haydn, from his years, no doubt, as music master to the Esterhazy family in Hungary. And in the final movement here Haydn lets go with a rollicking madcap frenzy.

The evening ended on a less eccentric note with Beethoven's "Ghost" trio. Its title derives from the harmonic mysteries and ambiguities of its slow movement, which was drawn out with great intensity by the Beaux Arts.