In the tightly constructed opening movement of the Trio KV 502, everything is based on a single theme—a very Haydnesque procedure but not so often a Mozartian one. It’s a lighthearted, rather nonchalant melody. (A little motif uttered by the violin does take on a role as a sort of subordinate theme, but the structural weight really is born by the principal melody.) As the movement unrolls we are surprised to find that the theme offers as much opportunity as it does for harmonic and contrapuntal elaboration.
The Larghetto is structured as a relaxed rondo, with the recurring theme seeming prescient of reflective songs in Schubert’s Die schöne Müllerin.
In the finale we find Mozart working out his material through a sonata-rondo form in which the principal theme not only recurs (à la rondo) but is also subjected to exploration through a finely wrought development section (à la sonata)—again with the unassuming melody harboring more possibilities for elaboration than we might have expected at first hearing.
The second and third movements of this trio seem not far removed from the piano concertos Mozart was writing at about that time, not only in their formal plans and the flavor of their themes but also in the way which the principal themes are adorned at their repetitions, not merely in the spirit of forthright decoration but as a means of expanding the emotional terrain.
Here is the second movement from this lovely trio:
