I was recently reminded of my dusty school days when someone asked me how jazz uses augmented sixth chords. Oh, I do so love these invitations to give a fag-packet explanation and get myself into trouble…
I suppose the simplest answer is that jazz uses augmented sixths all over the place, it just doesn’t think of them as augmented sixths at all.
The only real difference between an augmented sixth chord and a dominant chord is that the former carries specific expectations as to the direction the tones are to resolve. This chord is typically notated deliberately to highlight its “raised sixthiness” (which means using eye-catching accidentals on that note).
The harmonic function of the augmented sixth is as a pre-dominant chord, and one allowing for a lot of chromatic interest. The three classic variations (German, Italian and French) are as follows, preparing the arrival of G dominant:
Ge6: Ab C Eb F#
It6: Ab C C F#
Fr6: Ab C D F#
The augmented sixth chord appears on the bVI of the key, and the idea is that the lowest voice drops and the highest one rises, both by semitone, to the root of the following V chord an octave apart. The middle voices go where you’d expect, to B and D. The German one is highly organised and everything slips by semitone (how very German), the Italian one has the twin Cs resolving in different directions (how very Italian), and with the French one, the D is already there so stays put (how very French)…
What does a jazz brain make of this? Well, we’d probably think: big deal, that’s a ii V with a tritone substitution on the ii – Ab7 G(7). And we’d see the German and Italian ones as Ab7, the French one as Ab7#11. (There are supposedly Swiss, English and Australian ones too, but I’ve never really gone into them.)
So what a classical brain gets as an augmented sixth, a jazz brain gets as part of a tritone sub or “minor blues ii V”. And we pretty much resolve the tones in any direction we like, coz… well, that’s jazz. And we do anything we like with our dominants, thank you very much.
Augmented sixths can also precede I64 chords (that’s tonic over five to you) to extend a cadence. You come across this motion in certain early bluesy minor tunes – eg Ab7 Cm (with or without G in the bass). Again, a jazz mind would probably just call this a tritone-subbed ii I, and not worry very much about the sixthiness or nationality.
Later on, the classical chaps increasingly used the ambiguity of augmented sixth chords (sort of dominant and sort of diminished), among other things, to modulate wildly in all kinds of funky ways. This tendency eventually led to classical music becoming so chromatic that some people started fuming that it hardly even made sense anymore. Wagner usually gets the credit for this, but composers like Liszt were exploring this territory before him (the Sonata in B minor, for instance, foretold most of the 20th century stuff). You could argue that Beethoven got there even earlier.
There are lots of possibilities, but our jazz brains would recognise a lot of them as just going where the tritone subbed dominant “should” go. Faking it out. In other words, dominants freely changing identity mid-chord to whizz off into another key.
Right, I’ll put my tin hat on, ready for the comments…