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I have learned that the dominant seventh chord can be written with a flatted fifth, for example G-B-Db-F in C major. Recently, I heard the term tritone substitution and I looked it up on Wikipedia and here.

If I understand correctly, the tritone substition of G7 is Db7 in C. Can we consider the Db7 chord as being a G ninth dominant chord, with a flatted fifth and a minor ninth but with missing root, i.e. (G)-B-Db-F-Ab becomes B-Db-F-Ab to fit for four-part harmony?

So, can the Db7 chord people talk about enharmonically be the same with B-Db-F-Ab with a missing root (and with all the proper inversions)?

Last but not least, can the tritone substitution chord modulate up a tritone? For example, can the Db7 in C as a substitution chord of G7 be resolved in Gb and end the phrase in Gb?

  • With due respect, it seems as though you are regarding these sort of things as rules - 'can the ...' Anything in music 'can', and does. The ear is the best judge! There are few hard and fast 'rules', and most get broken at some point - and sound good despite that ! Try these things out - with different voicings - and decide yourself if they sound o.k. That's the real answer! As far as names go, usually a chord will have the root, that's where the name comes from. Could be considered otherwise, but simpler to call it including its root. – Tim Dec 21 '17 at 9:43
  • You're right. Of course it can. But the point of my question was more focused on the proper understanding and interpretation of the music theory concepts. I am still learning music harmony and I wanted to ask if my interpretation was somewhat correct. – George Dec 21 '17 at 9:56
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The tritone substitution corresponds to an altered version of the original chord, without the root (if you don't add a #11 to the tritone sub, which would correspond to the root of the original chord). You can add tensions to the tritone sub chord, and more often than not, these tension are not altered (except for the #11). These non-altered tension on the tritone sub correspond to altered tensions of the original chord:

tritone sub     original chord
     9             #5 / b13
   #11              root
    13               #9

The chord scale for the tritone sub is usually mixolydian #11 (lydian b7), which is the same as the altered scale over the original chord. E.g., in C you would play Db mixolydian #11 over Db7, which is the same as the G altered scale.

In sum, using a tritone sub is pretty much the same as altering the V chord.

As for modulation, of course you can use the tritone sub as a dominant chord moving down a fifth. So if you're in C, you could do | Dm | Db7 | Gbmaj7 |. I just don't remember hearing/seeing this very often.

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To extend this a little, it can be useful to consider a diminished 7th chord as any one of four different dom7b9 chords, with an assumed root a major 3rd below any of the notes. Hence, C#dim7 could (with an A root) be A7b7, (with C) C7b9, (with Eb) Eb7b9 or (with Gb) Gb7b9.

Not sure what you mean by the tritone substitute modulating up another tritone? That would bring it back where it started! But yes, a Db7 (however you arrived at it) can act as the dominant 7th of Gb.

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