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I'm learning a bit about chord progressions right now and so I stumbled across tritone substitutions. I tried applying those to a ii-V-I progreesion in G-major, but I'm not sure if I did it right, to be honest.

My basic progression looks like this:

Am7 - D7 - Gmaj7 - %

Now I want to substitute for the D7, where the tritone is F#-C. I did that by moving the root by a tritone from D to G#, forming a G#7.

The progression now looks like this:

Am7 - G#7 - Gmaj7 - %

Which seems about right to me (please correct me if this is not in fact a tritone substitution.).
The thing is, though, that this article calls the G#7 (G#-C-F#) a G#aug6th. Why is that? And why does this work, as the G# is not actually a part of the G-major scale?

Note: I know it's explained in the article I linked, but I don't get it, so please dumb it down a bit more.

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    Yes, what you have written is a tritone substitution. However, as a point of terminology, you should not be calling the tritone substitution here G#7 (G#, B#, F#), but rather Ab7 (Ab, C, Gb). Yes, they are enharmonically equivalent, but the standard practice is to consider the root to be a flattened second scale degree (bII) rather than a sharpened tonic (#I). For example, one benefit of doing it this way is that it emphasizes the common tone 'C' between Am and Ab, whereas G# has to be spelled with a B#. – Caleb Hines Sep 12 '14 at 15:32
  • Altering the question now would take your comment (and Tim's answer) out of context, so I'll leave it for now. I'll make sure to remember that in the future though, thanks :) – user13400 Sep 12 '14 at 15:41
  • To further elaborate, Ab7 is: (Ab, C, Eb, Gb); and Abaug6th would be spelled (Ab, C, F#) -- possibly with an additional note added, depending on the type of augmented 6th being used. – Caleb Hines Sep 12 '14 at 15:55
  • Maika, if you're reading this, we don't tag with our usernames. I rejected the edit because the tag added is just your username. Not sure whether you did that on purpose, but regardless, that's why I rejected. – user45266 Mar 26 at 17:24
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Playing a G#7 or a G#+6 is not really supposed to be in the G major scale/key. The chord is made up from G#, B#, D# and F#. The triad is standard, but the extra note, if the chord is called +6, will be Ex (E##). If it's called dom. 7th, that note becomes F# (as in Fx flattened). The B# of course being a C note on most instruments.

Tritone substitution works because in a V chord (D7 in the quoted case) contains a C and an F#, which is a tritone. It's pretty unstable and needs to resolve. However, G#7 (or G#+6) also has the same tritone - B# and F#. These need to resolve, and do so nicely to G maj7. Pretty well whatever key you're in, the tritone will look like a foreign chord, because it is. It's pivotal in that it can go either way.

In D, for example, the sequence Em7 - A7 - Dmaj7 will be t/s to Em7 - D#7 - Dmaj7. The A7 containing A C# E and G, while the D#7 contains D# Fx A# and C#. Or it may be called Eb7, with Eb G Bb and Db. All have that tritone between G (Fx) and C# (Db).

Basically, whatever it gets called, as far as a chord name is concerned, it's still a t/s. I agree, it's quite confusing. But if one just plays it rather than writing it down, the problem goes away. A bit like some guitarists who see an Eb triad as Eb, G and A#, for instance...

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With due respect to the previous posters, I don't think the question of why the chord would be referred to as a G#aug6 was really addressed. Let me have a go...

Consider the progression F/A to G. One nice way of scoring it would be as follows:

    F -> G
    C -> B
    A -> G

That works well because no note moves by more than one step, and there's some contrary motion to add interest. But it could be more interesting. How about we add a bit of chromaticism:

    F -> F# -> G
    C -> C  -> B
    A -> Ab -> G

Chromaticism just means using notes which don't belong to the key. In this case we raised the F to an F# to create a smoother movement to the G above it, and lowered the A to an Ab in the direction of the G below it. In doing so we have created a new chord made up of the notes Ab, C and F#. Because the interval between the Ab and the F# is an augmented sixth, we call this, rather imaginatively, an augmented sixth chord.

This is how tritone substitution first came about, way back in the days before jazz. It wasn't thought of as tritone substitution, but as an altered 6-3 chord (a chord which features a sixth and a third above the root, i.e., as in this example, A-C-F, altered to Ab-C-F#). Anybody using this naming system presumably learned their theory the old way...

Over time, as composers pushed the boundaries of musical theory, this chord was used more and more often 'unprepared', for example without being preceded by the A-C-F chord in the previous example. It was also enhanced with the addition either an augmented fourth, or perfect fifth, i.e. D or Eb in the previous example.

By the time jazz theory came to be formulated, the old rules of music had been eroded to the point where it no longer seemed relevant to refer to a chord which contained the exact same notes as a seventh chord as anything but a seventh chord.

In conclusion: Ab7 for jazz, Abaug6 for classical music and, as has been pointed out previously, never G# anything.

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