Playing piano duets, or "piano, four hands" as programs usually put it, is a very sociable thing to do. Not only are two people playing the same music, they are playing the same instrument, elbow-to-elbow, pinky-to-pinky. They better be friends.

Leonard Klein and Joseph Rezita, who performed at the Phillips Collection yesterday, are good friends and their playing showed it. Their style is convival and warm, fine technically but not flashy. With their casual manner they could have been playing in someone's living room.

The program was well calculated to give a varied view of music written for this medium. The frothy Satie "Trois Morceaux en forme de Poire" is pure salon music, witty, sharp and pretty, the ultimate in sociable music-making.

In his waltzes Opus 39, Brahms not only displayed the infinite varieties of the waltz, but the many ways the piano's horizons can expand and still remain pianistic.

Klein's own Sonata (in its first Washington performance) was written to explore both the possibilities presented by 20 fingers at the keyboard, and the fun of tricky coordination. It sounded it. And Weber, in his eight-piece Opus 60 used the piano symphonically.