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Suppose we are in the key of C:

Ab7 would be the "augmented-six" chord.

But doesn't any 7th chord have the same intervals as the bVI7?

Why does bVI7 only have the particular name of "augmented six"?

Is it because bVI resolve to the V (because its only half step above)?

  • Aug 6 from C is A#. Maybe A#/Bb7 is what you mean? C >Ab is a m6 interval. – Tim Feb 28 at 11:23
  • @Tim - I think Hyun Yoo Park means that Ab7 is (enharmonic to) an augmented 6th chord in keys with a C tonic. Keep the 5th and it's enharmonic to a German Augmented 6th; drop it and it's enharmonic to an Italian Augmented 6th. – Dekkadeci Feb 28 at 12:02
  • I was talking about this en.wikipedia.org/wiki/Augmented_sixth_chord – Hyun Yoo Park Feb 28 at 12:52
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An augmented sixth chord most often functions as a predominant. When it does so, it is enharmonic with (or at least very similar to) the ♭VI7 chord. In the key of C:

  • The Italian augmented sixth has A♭, C, and F♯.
  • The French augmented sixth has A♭, C, D, and F♯.
  • The German augmented sixth has A♭, C, E♭, and F♯.

We call these chords augmented sixths and not ♭VI7 chords because of how they are spelled: the interval from A♭ to F♯ is not a minor seventh, but an augmented sixth. And because of that spelling, these pitches resolve differently: in the augmented sixth interval the F♯ will move up to G, but in a ♭VI7 the G♭ will want to move down.

It's important to note that not all augmented sixth chords function as predominants. And in at least one particular instance, this non-predominant augmented sixth chords is not enharmonic to ♭VI7:

Lastly, an augmented sixth chord can also function to just expand tonic; this is often done by means of what we call a common-tone augmented sixth chord. These are relatively rare, but normally these result from the chordal fifth of tonic moving in contrary motion by half step. In C, we begin with C E♭ G; if the G then splits and moves outward by half step to A♭ F♯, we're then with an augmented sixth that is spelled "correctly" but doesn't function as a predominant. I discuss this chord in Analyzing Tschaikowsky's "Wintermorgen 2"

  • What do you mean by "French augmented sixth is enharmonic to a V7 chord with a lowered chordal fifth" The notes are totally different – Hyun Yoo Park Feb 28 at 13:40
  • @HyunYooPark Good question! I was unclear in my post, which I've now edited. In short, the French augmented sixth in C has A♭, C, D, and F♯. This is actually enharmonic to two dominant seventh chords with lowered fifths: one built on A♭ (A♭, C, E♭♭, and F♯) and one built on D (D, F♯, A♭, and C). – Richard Mar 29 at 19:35
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The Augmented Sixth is characterized by the outward resolution of an augmented sixth (hence the name) to an octave. This intervallic resolution dates back to the Medieval Era. The resolution to the octave consists of two half-step moves. The particular forms (termed Italian, French, and German, for no particular reason) combine the double half-step resolution with a tritone resolution (or two in the case of the French Sixth). This combination is enharmonic with some (perhaps altered) dominant seventh resolutions but the voice move differently from that of a dominant chord.

There are several possible derivations but composers seem to treat the Augmented Sixths as objects in themselves rather than altered chords. The most common location is on the flattened sixth step which leads to V chord (perhaps through a I64 to avoid consecutive fifths and octaves if necessary), however there are others. An augmented sixth on the flattened second step does occur throughout the 1650-1900 time frame; it resolves to the tonic. There are secondary Augmented Sixths which resolve to a secondary dominant. The German Sixth is the dominant of the Neapolitan Sixth so these chords can occur consecutively in either order. The Aug6-V movement is so strong that it can signal a modulation without playing the new tonic immediately.

The French Sixth has two tritone resolutions in addition to the resolution of the augmented sixth; it is enharmonic to a dominant seventh with a lowered fifth. The German Sixth is enharmonic to a dominant seventh. Both chords can be resolved as either. In some jazz pieces, the tritone resolution is treated as more important than the augment sixt resolution leading to the idea of tritone substitutes. The difference is in the resolution; chords are identified by their resolution (the sound of the music) rather than by their labels or spelling. Some classical and romantic composers approach the augmented sixth as an augmented sixth and leave as a dominant seventh (of a chord a tritone away from the other resolution); the other way around is also used. These are useful for making smooth connections between chords which don't share any tones.

In Classical and Romantic era music, the Augmented Sixths are used as pre-dominants. It seems that one can never have enough pre-dominants to choose from so that the dominant resolution can be delayed. In the cadential pattern ii-V-I or ii0-V-i (or II-V-I or ii65-V7-I, or any of these), an Augmented Sixth can be placed after the ii chord to extend the cadence. One can combine these with a Neapolitan Sixth to extend things further (and through a I64 in just before the V for that matter.) Sousa's marches tend to have a German Sixth somewhere.

The use of tritone substitution is different and I don't know use it enough to comment.

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I think you are misunderstanding what the root of the augmented sixth chord actually is.

Enharmonically the German augmented sixth chord seems to be a root position dominant seventh chord. By this perspective you are thinking the root is Ab and then there is a minor seventh above it at Gb. Your full spelling is then Ab C Eb Gb which is root position chord.

The problem with that is where is the augmented sixth?

In actuality the augmented sixth is from Ab to F#! This gives us the correct spelling of Ab C Eb F# which is a first inversion seventh chord with one very important additional characteristic_: the root is chromatically altered!

Rather than the chord being rooted on Ab it is rooted on F#. F being the subdominant degree in the key of C, it's actually a chromatically altered subdominant chord.

If you write that as a Roman numeral analysis symbol it should be #iv6/5.

While that augmented sixth chord as the sonority of a dominant seventh chord it doesn't resolve like a dominant seventh.

With an actual dominant seventh in the key of C the root of the V7 (G7) chord will be G and it will be held as a common tone when resolving to i.

In the case of the augmented sixth chord the alter fourth - the F# - is not held and has a strong pull to resolve up to the dominant scale degree G. That particular movement makes clear the subdominant function of augmented sixth chords. It resolves to the dominant.

...the same interval as the bVI7

No. In C minor the chord would be Ab C Eb G with a G natural which makes a major seventh chord. That different should help underscore that the German augmented sixth chord is not a type of bVI chord.


You may be interested to know there is another augmented sixth chord spelled Ab C D F#. Called the French augmented sixth chord.

The root of that chord is D which makes is a chromatically altered supertonic seventh chord. You could use the Roman numeral symbol ii#6/4/3 for it.

In the jazz world both of these chords can appear in enharmonic respellings as tritone substitutions.

The German augmented sixth as a tritone substitution could look like bII7 as in Dm7 Db7 C6.

The French augmented sixth as a tritone substitution could look like bII7b5 as in Dm7 Db7b5 C6.

In that jazz context notice that the chords are substituting the dominant chord V and that would be the rationale for why they are enharmonically respelled and don't fulfill the subdominant function of the alternate spellings of the German or French augmented sixth chords.

The important take away point is: enharmonic spellings should reflect how the chord is functioning!

  • There's also the Neapolitan +6 chord, a personal favorite. I would underscore the classical voice leading, which is what helped me to understand +6 chords when I finally did understand them. The +6 resolves outward to an octave. In the example, the F# moves up and the Ab down, both to G. The minor seventh typically resolves down by half step (and in practice, a +6 can do that too, usually for reasons of crunchy harmony, but it's not the classic +6 paradigm). – phoog Feb 28 at 17:12
  • Do you mean where the chord would resolve to a tonic chord instead of a dominant? – Michael Curtis Feb 28 at 17:17
  • No, we're in the key of C (or, more likely, C minor). If the dominant on G is a dominant seventh chord, then the voice with the F# of the +6 chord might move to the F natural of the V7 chord, though it should really resolve to the G. In any event, the classic progression is more likely to interpose second-inversion tonic chord between the +6 and the V7. (The first sentence is not particularly related to the rest of my previous comment.) – phoog Feb 28 at 17:21
  • Well sure, that's all covered in a textbook review. But why are you calling it a Neapolitan? That would seem to imply a chord rooted on the lowered supertonic. – Michael Curtis Feb 28 at 17:35
  • Phoog // That's not even an augmented sixth. It's a minor sixth chord that is half above the tonic. No one would even call a neapolitan an A6 chord! – Maika Sakuranomiya Mar 8 at 4:33
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In a piece in C major, we are very likely to come across D7, E7, F7, G7, A7...and more... (Yes, a chromatic chord or a secondary dominant doesn't have to mean there's a change of key), and they're all 'dominant 7th shape' chords. But only G7 is THE dominant 7th of C.

An Augmented 6th chord is classical harmony's nervous first attempt the idea of a 'b5 substitution'! There's a special name for the case where it's rooted on the b6 and moves to the dominant - 'Augmented 6th'. You can of course use a tritone substitution for any dominant 7th shape chord that CONTAINS a tritone. But the label 'augmented 6th' is generally reserved for that particular one.

  • One person's "nervous first attempt at..." is another person's "innovative development that led to..." But I think Michael Curtis has it better by pointing out that the +6 originally arose in a subdominant function. That it led to tritone dominant substitution seems more to be a result of the similar position that may be occupied by subdominants and secondary dominants. – phoog Feb 28 at 17:13

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