Although the C-major quintet is accepted as one of Mozart’s greatestworks, it is not generally recognized as perhaps the most daring ofall,” wrote Charles Rosen in a discussion that no one who has sincethought about this work can ever put out of mind. I want to pursueRosen’s claim with some observations about a few notes in the midstof the first movement of the work, where the daring, to my ears, is mostdeeply felt.

The passage that I have in mind is a famous one, but identifying the momentat which it begins—testimony no doubt to the seamless unfolding ofthought and idea in this remarkable work—is no simple matter. Atmeasure 170, the cello finally joins a cadencing in A minor that had begun afew bars earlier. Its deep E might at first suggest the incipit of anotherof those grand arpeggiations that have been setting new paragraphs in motionsince the opening bars. Of course it is nothing of the kind, but rather theincipit of a fugal subject. Here, too, the identity of this subject at itsinception and its close is intentionally complicated. The subject means torecall the expansive closing bars of the exposition, music of perfectequipoise, spun out over what seems an endless pedal on the new tonic, Gmajor. The phrasing of this closing music is not quite so simple as it mayseem. The whole note at measure 131, emphatically the final note of thestructural cadence before the closing theme, has a Janus-like aspect to it,triggering this lavish epilogue of afterbeats. Here too, the whole note isambivalent, for while the downbeat at measure 131 sets up the first of aseries of four-bar metrical units, the new theme itself begins only atmeasure 132, suggesting a down-beat that cuts across the larger metricalbackground, oblivious of the whole note G that sets the passage in motion(see example 4.1).

At the inception of the fugue subject, this whole note now seems to reexaminethe ambivalence of its articulative function at measure 131.