Form

sonata - built around the elements of a double (or even a triple) fugue (fugato)

Theme (G major) - 1st fugato subject

Theme (D major) - 2nd fugato subject

Theme (D major)

3rd fugato subject - exposition, development and coda

Notes

This movement is one of the most amazing in all of the string quartet literature. It combines the sonata form with the process or procedure of a fugue. The sonata is a dramatic plan for the entire movement: it stipulates a basic formal organization around the use of themes and key areas for harmonic motion, development and resolution. As thematic material within this form, Mozart uses the principles of the fugue for structuring the melody, texture and ensemble of the quartet. But the fugue is not constant: it is fragmentary, coming and going, intersperced with thematic sections more characteristic of the classical sonata (e.g. lyrical themes rather than short subjects, comparatively more homophonic textures which are not strict counterpoint). While is might be more correct to use the term "fugato", fugue-like writing within the context of a non-fugue composition (e.g. a classical sonata), the constant recurrance and even development of the fugue overtime gives the fugato writing such integrity that is stands in bold relief, nearly a fully formed fugue embedded within a sonata. This impression is strengthened among other things by the fact that it has three different subjects, two of which combine to create a double fugue.

The blend between classical sonata and the learned counterpoint of the fugue make this movement a novel form. It is much less rigid, much more hybrid, than the earlier use of not just fugato but complete fugue in Haydn's Op. 20 quartets. And yet the clear and relatively undisturbed shape of the classical sonata keep this movement more light and in a sense restrained than the later fugal expeditions of Beethoven. As a composition uniquely suited to the string quartet ensemble and the emerging genre of string quartet rhetoric in the classical era, this movement is supreme. It is not only ideal for the string quartet; it defines the string quartet ideal.

What might be considered the 3rd fugue subject appears in isolation at the beginning of the development and the coda. It begins, in fact, in the last measure of the exposition, but it is not recognized as other than a simple chromatic leadin if and when the exposition is repeated. That it is a subject is clear from that fact that it appears at least nine times in succession during both of its appearances. But since it does not combine with either of the two more prominant subjects in the exposition, it is not necessarily part of the same "fugue". It is perhaps no more than additional fugato writing which Mozart, in his inexhaustable imagination, choses to lend greater textural unity and wit to the sonata. Or maybe one could say that this sonata features two different fugues, one double, one single. But with all its short chromatic simplicity, this 3rd subject, more importantly, appears to function as an inter-movement linkage. It seems to recall the chromatic ascending lines in the 1st and 3rd movements thereby stitching the movements together across the quartet, much like the fugue and sonata are stitched together within this single movement. In this sense, the fugato writing seems to finger its way through the sonata and out into the adjacent movements. It is presaged. Or, again, it simply smiles knowingly back on what came before it. It is a small detail, without the dramatic effect of so much else throughout this quartet. But it is characteristic of Mozart's genius, a small and brilliant signature that he inscribes on nearly every quartet in the set of six dedicated to Haydn.