Anyway, as I was trying to think of a favorite augmented sixth chord to feature, I suddenly remembered some magical Mendelssohn this same daughter's young string orchestra performed six years ago. I wrote about that music in this blog post, trying to explain all the reasons why a low-quality recording of kids too young to know better is THE definitive recording of the slow movement of this particular string symphony Mendelssohn wrote when he was 12(!).
I mentioned briefly in that post that THE most magical moment in this movement occurs in the transition to the recap. The music has meandered from C Major into the relatively distant key of A-flat Major and in a classic "treading water" motion, the second violins and viola are slowly arpeggiating in A-flat as if lost and wondering where to go. In the third measure below, the harmony changes to C Minor* over the same A-flat in the bass (creating an achingly lovely major seventh sonority) and then the G changes in bar 4 to an F-sharp, so that we have the classic augmented sixth interval between the bass (A-flat) and F-sharp, and in classic augmented sixth fashion, this German Sixth chord resolves outwardly by half-steps, with the A-flat heading down to G and the F-sharp leading up to G. G is the dominant in C Major, and suddenly we're back in C Major with the opening theme back in the right key, although beautifully poised above the expectant dominant G in bass rather than tonic C. It is all SO much more sublime than I've just made it sound.
[The audio excerpt begins about 5 seconds before the score excerpt above.]
And there you have it. I'd love to say more, but I've gotta pack the car!
* This is really better understood not as a C Minor harmony, but as the A-flat in the second violins descending down chromatically to a G in bar 3 above and then to the magical F-sharp.