6

So, if you're playing in a major key, say C major, and you play a I IV V chord progression but before you go to the V you play a bVI7 chord. So, C Major, F Major, Ab7, G Major. How come the Ab7 has such a strong pull to the V chord?

7

It's called "tritone substitution". The notes of Ab7 are Ab, C, Eb, Gb (or F#). This chord has two notes that are also present in the secondary dominant of G, ie D7 - C and F# - and Ab and D form a tritone (as do C and F#). Basically one can often replace a secondary dominant by the chord formed on its tritone.

7

It is known as tritone substitution. It works because two notes out of the four are the same as two notes from the actual dominant of the next chord played.

Let's look at the notes: G (the target chord) has D7 as its dominant. D7 contains D F♯ A and C. Its tritone sub. which is A♭7 contains A♭ C E♭ and G♭.

It's always the 3rd and 7th that become 7th and 3rd. So 3rd of D7 is F♯, 7th of A♭7 is G♭ (both enharmonic in 12tet). And - the 3rd of A♭7 is C, and the 7th of D7 is C. So the two notes kind of swapped over. That works sonically so using A♭7 instead of D7 gets us to G.

The two chords contain that tritone, which is instrumental in pushing towards consonance by being one semitone from notes in the target chord. The chords themselves (D7 and A♭7) are a tritone apart - look on the circle of fifths, they're diametrically opposite, (thus three tones from each other), so what better term to use!

1
  • 2
    And also it allows a chromatic downward movement in the bass – a key feature of why the sub works so well!
    – Judy N.
    Jan 12 '21 at 12:37
5

The Ab7 chord is considered an Ab augmented sixth chord, spelled [Ab C Eb F#]. The "standard" resolution of that chord would be to [G C E G] followed by G major, but it's acceptable in modern music (i.e., 20th century and later) to just go directly to G major.

The key concept behind augmented sixth chords is that the augmented sixth ([Ab-F#] in this case) resolves "outward" to an octave. As opposed to a dominant seventh chord (containing [Ab-Gb]), in which the seventh (Gb) resolves down by step (to F).

Theory describes dominant seventh chords as being V chords of the related key; whereas, augmented sixth chords are bVI chords in their respective key. Thus Ab7 would be expected to resolve to Db (major or minor); while AbAug6 would be expected to resolve to G (by way of a [G-C-E] chord, which some people call I[6-4] and others call V[6-4] or a "cadential 6-4 chord).

See also: Why does the A♭–C–F♯ augmented sixth chord resolve to G?

10
  • 1
    What advantages do you see the "augmented sixth" explanation to have over the "flat-five substitution" explanation (which is also the accepted answer in the linked question)? The reason I ask is to me it seems to overelaborate the situation by introducing a "rare" chord with a lot of finikity classical rules about how it's "allowed" to be used, where the flat-five account covers exactly the same ground and describes exactly the same harmonic behviour without me imagining harpsichords and powered wigs when I read it!
    – Judy N.
    Jan 12 '21 at 12:30
  • Regardless of dominant seventh vs augmented sixth chord, the key is the common tritone (in equal temparament) and the key word is "tritone substitution" mentioned in the other answers. While the augmented sixth interpretation can be justified as above, I think it's fairly common to call any major chord with a minor seventh a "dominant seventh chord", regardless of its resolution in a given context. The augmented sixth interpretation simply avoids calling something a "dominant" that doesn't resolve to a fifth down. But since it clearly has secondary dominant function here, I don't mind at all.
    – Streck0
    Jan 12 '21 at 13:06
  • @Streck0 - I agree. I think(!) the chord will still be a secondary dominant, as sec.doms don't have to be followed by their chord a fifth below. Sec. doms just have to have non-diatonic notes.
    – Tim
    Jan 12 '21 at 13:14
  • 2
    @JudyN. The benefit of the "augmented 6th" explanation is that it is part of common practice period harmony. Thus, advanced enough regular music theory classes (e.g. the Royal Conservatory of Music Harmony syllabus) will bring it up instead of the "flat-5 substitution" explanation. As a bonus, the same theory class will also explain that Ab-C-D-F# (French Aug. 6th) also similarly resolves to a G chord just fine.
    – Dekkadeci
    Jan 12 '21 at 13:51
  • @JudyN. the answer explains it quite clearly – an aug6 chord behaves observably different from a dom7 one, in that the 6+ leads upwards, while a ♭7 generally leads down. In other terms, the tritone is “flipped”, i.e. the interval between the third and the “seventh” is actually an augmented fourth rather than a (diminished) fifth as it would have to be for a seventh chord. Jan 12 '21 at 14:13
2

I remember that when I didn’t know anything about augmented 6th chords or tritone substitution and borrowed chords I was already playing and experimenting with this progression.

Now, beside the given answers I think it’s worth to recall that Ab7-G is borrowed from the parallel key C-minor, and reminds us also on the Andalusian cadence, which everybody has in the ear, whether musician or not.

4
  • Now that's an interesting slant on things!
    – Tim
    Jan 12 '21 at 19:59
  • 1
    I'm inclined to think that all explanations are "true" at the same time. Whichever way you see logic there, if it enables you to reason about the situation so that the logic applies to other similar situations, then that's a working theory and "true". Jan 12 '21 at 21:45
  • 1
    At least that’s what I think what examinators expect of a student (not only in a music exam): that he is able to render a theory and arguments to defend it. Too often we think there must only one right, correct answer. Jan 12 '21 at 22:32
  • It's true that most of us have stumbled on something musically, well before finding that the theorists beat us to it with explanations. Serendipity!! +1.
    – Tim
    Aug 25 '21 at 7:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.