When I signed off last week I was wracking my brains trying to think of the name of the jazzy tune prefigured in the finale of Mozart’s K. 590 string quartet.  My thanks to reader Doreen Spungin for passing along her son Andrew Lubman’s guess as to the tune in question, which 1) turned out to be absolutely correct and 2) allowed my life to resume a less troubled course.  It’s “Twelfth Street Rag.”

Mozart quartets being on my mind, I got to thinking of a remarkable spot in the 3rd movement–a minuet-and-trio–of Mozart’s immediately preceding one, K. 589. But in order to realize what’s remarkable about it, you have to know a little about how minuet-and-trio movements are normally put together. In simplest terms, a movement of this kind is in an A-B-A form: a minuet (itself in two sections, each of which is repeated), a “trio” (some completely different music, also in two repeated sections), and a return of the minuet.  One more thing: the trio is usually more modest, in every respect, than the minuet: “lighter,” more naive, easier-going–and often just plain shorter. A good example of the typical trio’s relative lightness is found in the minuet-and-trio of Mozart’s “Jupiter” Symphony.  The trio begins at 2:30 in this clip. Notice how much shorter and less grand and complex it is than the minuet by which it’s sandwiched. (It does have one intense stretch, but things return in short order to sweetness.)

Now look at the minuet-and-trio in the K. 589 quartet.  The minuet is a rich, noble, and–in its second part–expansive item. The trio comes at 2:00 in the clip–and with it, what can only be called, to use the technical term, a switcheroo.  Because it’s the trio (which here, unusually, is much longer than the minuet) that’s the really ambitious element in this movement–especially its second half (beginning at 3:07) with, as its capper, a marvelous extension, beginning at 3:51, that features virtuosic cross-bowing in the first violin. Just when we’re expecting the movement to become more relaxed–to take, in its trio, a kind of breather–Mozart launches into one of his most extended, exciting, and inspired flights.  That’ll teach us to think we can ever be sure what genius has up its sleeve.