The string trio – violin, viola and cello – has a much smaller repertory than the venerable string quartet, until you add one more non-string instrument to make a quartet. There is a large repertory for piano quartets.
Neither Haydn nor Beethoven nor Schubert wrote in this genre. Beethoven’s Piano Quartet in E-flat is a reduction of his Quintet for Piano and Winds in E-flat, Op. 16, for the home market. You have to go to Schumann and Mendelssohn to find more piano quartets, for it was then that the genre blossomed and grew.
Schumann no sooner finished with his piano quintet in this key than he started a new piece. It starts out slow in 4/4, laying out a germ motif that will dominate the movement, a motif that sounds a lot like the first bars of the Finnish national anthem. Then it takes off in 2/2 with one of his catchiest melodies, which he begins to develop even in his exposition. In fact, there is so much development going on that it’s hard to find the exact moment that he slides into B-flat for his second subject.
It comes to a screeching halt when he brings back the introduction, and then he charges into his real development section in the minor keys. This section is so tonally unbalanced that it looks ahead to Wagner, and there is no sense of genuine key until the recap in E-flat is reached. That comes as a relief, finally permitting the listener to find a tonal reference point.
A bridge passage in A-flat and C minor permits Schumann to place his second subject in the correct key of E-flat. The coda slows down a bit for a fast wrap up with a loud, decisive E-flat on the piano ending it.