An Argument in Three Parts

The usual account of sonata form goes: first theme, second theme, development, then both themes again. Which makes it sound like a mould that composers poured music into, and explains why so many people find classical music tedious to be taught.

The account is wrong in two ways worth separating.

The first is historical. The form was codified by theorists in the 1830s and 1840s, most influentially by Adolph Bernhard Marx, describing works written fifty to eighty years earlier. Haydn and Mozart were not following a rulebook, because the rulebook was assembled from what they did. They worked within conventions they shared with their audiences and varied constantly, which is exactly why the pieces most worth studying break the rules that were later derived from them.

The second is musical, and it matters more. The themes are not the content. The keys are.

Here is the shape. The exposition states material in the home key and then moves to a contrasting one, traditionally the dominant, or the relative major if the movement is in a minor key. That move is the event. The music has left home and established itself somewhere else, and an eighteenth-century audience heard that as an unresolved situation.

The development takes the material apart. Motifs are fragmented, recombined, passed between instruments and pushed through keys without settling anywhere. This is the least stable part of the movement, and its instability is functional: it is what makes the return feel like a return.

The recapitulation restates the material, and here is the point everything has been building towards; the second group now appears in the home key. The tonal conflict set up in the exposition is resolved by absorption. What was foreign is brought home.

That is the drama. Not two nice tunes and a middle bit, but a departure, a period of maximum instability, and a resolution, with the tonality carrying the argument.

Once you have that, two things follow that the textbook version cannot explain.

A movement can have one theme and still be a sonata. Haydn does this repeatedly (the 'second subject' is the first subject in the new key) and it is completely coherent, because the opposition is tonal rather than thematic.

And a movement with two lovely contrasting tunes, a middle section and a return, all in one key, is not a sonata. It is a ternary form with contrast in it. Perfectly good, and a different thing, because the conflict that the sonata exists to stage never occurs.

The analogy with dramatic structure is not decoration. An opposition established, intensified and resolved is what allowed the form to carry increasingly long and serious music for a century and a half, from movements of five minutes to Beethoven's of twenty, and it is why the same shape kept being used long after the style around it had changed beyond recognition.

For a listener, the practical instruction is short. Do not try to spot the themes. Listen for the moment the music stops sounding settled and starts sounding as though it is somewhere else, and then, much later, for the moment it comes back. Those two events are the piece.