Those wishing to get to know Mozart's gentle, slightly elegiac side should play the opening movement of this lesser-known piano sonata K. 570. The second movement, too, marries the calm strains of horns with a somber melody in C minor. Only in the spirited B-flat major finale (''Allegretto'', in alla breve meter) does the mood shift to - restrained - gaiety. Strangely enough, this sonata was initially published (posthumously in 1796) as a violin sonata, a version that is not at all hard on the ears. If we didn't have Mozart's autograph manuscript of the piano sonata (albeit only in fragments), along with his entry in his own catalogue of works (Vienna, February 1789: ''A sonata for piano solo.''), one might presume that this sonata in B-flat major existed as a genuine pair of twins (with and without violin accompaniment).