The augmented sixth chord(s)

[Jean-Michel G]

Suppose we have a melody that is harmonized with the following cadence:
|C – – – |F – – – |G – – – |C – – – |
This is of course the plain old I IV V I in C major.

We can also harmonize it as follows
|C – – – |F/A – – – |G – – – |C – – – |
Where the IV chord appears in first inversion: F/A = (A C F).

Now let us look at that from a pure voice leading perspective; we can smooth out the path from A to G by inserting an Ab in the bass, and we can similarly smooth out the path from F to G by inserting an F# in the top (soprano) voice.

The resulting passing chord (Ab C F#) contains an interval of a #6 between the bass and the soprano; it goes by the name “Italian augmented sixth” chord.
So we get:
|C – – – |F/A – It+6 – |G – – – |C – – – |

In fact, there are two other variants of this chord: the “French” augmented sixth (Ab C D F#) and the “German” augmented sixth (Ab C Eb F#).
Those names don’t mean anything. They are pure inventions from an obscure English music theorist of the 19th century!

This chord was rarely used during the Baroque period (although J.S. Bach did use it), but it was used a lot starting from the Classical period (Mozart used it a lot). In the Romantic period is was even used as a chromatic approach chord towards other chords than the V chord.
Since this chord follows a predominant chord and precedes a dominant chord, it has a predominant function itself.

It is still used today…
In fact, if we think “enharmonically”, we can respell all these chords:
It+6 = (Ab C F#) = (Ab C Gb) = Ab7 without 5th
Fr+6 = (Ab C D F#) = (D F# Ab C) = D7(b5)
Ger+6 = (Ab C Eb F#) = (Ab C Eb Gb) = Ab7

So this chord is enharmonically equivalent to either an altered D7 or an Ab7, and Ab7 is a tritone substitute for D7.
But then in the cadences
|C – – – |F/A – D7 – |G – – – |C – – – | or equivalently
|C – – – |F/A – Ab7 – |G – – – |C – – – |
the D7 and Ab7 chords are secondary dominants, not predominants!

Most contemporary musicians, particularly jazz cats, actually look at the augmented sixth chord as a tritone subs… But is it really? The internet is (of course) full of very hot debates about this.

I will not take a stand. I will only make the observation that an augmented sixth chord is built around the interval of augmented sixth, which resolves outwards, whereas the dominant seventh chord is built around the interval of a tritone, which resolves inward.
Put differently, it is possible to build an entire family of chords containing that augmented sixth, and an entirely different family if chords contain the tritone.

I guess the main point of this post is to stress, once again, the importance of melodic thinking and voice leading, since the augmented sixth chord really stems from a pure voice leading approach and not from a harmonic approach. Jazz and bossa nova musicians often insert such chromatic passing chords without worrying much about how to call them!

[Andy N]

I came across the Ab7 substitution for D7 in Robben Ford’s rhythm course. Until now I just knew it worked but now I’ve got a much better understanding of why.
These chords do sound very cool when played at the right point and I get what you’re saying about the voice leading being a big part or rather THE part of getting that right.
This definitely something I’m going to have a play with.
One question though.
When you say Fr+6 is the same as D7(b5) that is just another name for the same chord in the same context? Rather than (say) an A6 has the same notes as F#minor/A but are different chords and depending on the context they are played in?

[Jean-Michel G]

Well, that’s the question, I suppose…
In my personal opinion, they are two different chords that achieve the same effect (resolving to G), but in two different ways, depending on how you voice the chords.

Let’s start with the Fr+6: (Ab C D F#)
The important chord tones are the Ab and the F#, and both resolve outward.
The Fr+6 chord is essentially a passing chord.

If we respell the Fr+6 chord enharmonically, we get Fr+6 = D7(b5)/Ab; let’s voice this chord (Ab D F# C).
The important notes here are the F# and C (the tritone), and they resolve inward.
The D7alt/Ab chord is essentially an altered dominant chord whose tritone resolves as it should.

From a purely practical perspective, it doesn’t make any difference, but from a voice leading perspective it does. Not all the possible voicings will sound equally good.

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