Qfyilyi

Is is possible to use tritone substitution in modal chord progression?

For example I have a simple I-IV-V-I chord prog, in C mixolidian:
C-F-Gm-C.
Can I substitute the Gm with Db7? And if yes the functionality will remain the same?

I've read somewhere (i cant remember where) this work a little bit different. In this case I want to substitute the dominant chord, so according that I have to calculate the dominant chord's replacement chord in the major scale which is F major. In F major the dominant note is the C and its tritone is Gb. So It would be C-F-Gb7-C.
It seems wrong to me.

Thanks in advance.

...I-IV-V-I chord prog, in C mixolidian: C-F-Gm-C...

You're mixing up symbols and tonalities.

If C mixolydian, then the V - the triad on the dominant/^5 scale degree is a minor chord, so lower case v.

        C-F--Gm-C

C mixo: I-IV-v--I

If we speak properly of a dominant chord then the chord must have a leading tone the ^7 scale degree one half-step below the tonic.

Mixolydian mode doesn't have a leading tone, therefore it doesn't have a dominant. Strictly speaking then you don't have a dominant to substitute.

Side note: to me modal generally implies music without a real dominant chord. Ionian mode of course it just the major mode so that more or less excludes Ionian from the general sense of "modal" harmony. Except for the Lydian mode all the other modes do not have leading tones and so they don't have dominant chords. From that perspective tritone substitution may be foreign to modal harmony.

Back to your chords.

If you change your harmony to use a real dominant you will be in C major, you would get...

   C-F--G7-C

C: I-IV-V7-I

With labels

   C-F--Db7--C

C: I-IV-bII7-I

I suppose you could have that G7-C change and still have the music sound like it was Mixolydian, but it would depend on exactly how you handled G7-C. It would need to sound like a temporary tonal shift. It would help if the melody somehow clearly sounded Mixolydian. Perhaps by avoiding the raised ^7 degree. You would have to handle the harmony somehow to make Mixolydian the clear tonality and the bII7-I substitution seem temporary. You would probably need the minor v in the progression to make Mixolydian clear. You would have to play with that Mixolydian/major and B♮/Bb relationship.

Finally, the other way to look at this is to say your progression isn't really in C Mixolydian, but rather in F major. In that case you have to re-label your chords...

   C-F-Gm-C

F: V-I-ii-V

We have a proper dominant now, but it's in a different position in the order of the chords. The substitution might be like...

   Gb7--F-Gm-Gb7

F: bII7-I-ii-bII7

Back to part of your question...

...In F major the dominant note is the C and its tritone is Gb. So It
would be C-F-Gb7-C. It seems wrong to me...

You were correct. The only problem is you substituted the wrong chord! You substituted the ii instead of the V chord.

So, make sure you identify a real dominant chord then make the tritone substitution.

The functionality will remain the same, as ♭II7 can be resolved to I the same way your Gm chord would be. However, you will lose the modality of the progression, because ♭II7 doesn't imply that mixolydian sound you have going when using the Gm. Also, Tim pointed out that the Gm might want to resolve to C7, then to F, but I think that that would depend on how you're using it, as it's certainly possoble to go from Gm to C without going to F.

Tritone substitution relies on the juxtaposition of the 3rd and b7 of two chords. So, in key C, the dominant G7 will have Db7 as its tts. G7 is G B D F, whereas Db7 is Db F Ab Cb. So, the F changes place (3rd>7th), and the same sounding B/Cb goes the other way.

With your suggestion, there's only one common note between Gm and Db7. That's the F. So it's not really a tts.

If anything, to me, that Gm>C sounds like it's heralding a C7 to move onto subdominant F, and the 'tts' version throws a spanner in the works, so to speak!

Whilst you're correct in saying the scale is F, the sequence is not in F, although it is using the same pool of notes. In C Mixolydian, the home or root is C, that's the point. At the end of the day, anything is possible - there are no hard and fast rues saying 'thou shalt/ shalt not...' If you think it sounds good, keep it. If not, it was worth a try, but not this time thanks.

But, bottom line is, as said earlier, tts needs those two notes common. That's what makes it tts.

Yes, Db7 acts as a dominant of C. We call it a 'tritone substitution' partly because its root is a tritone from the 'real' dominant - G, but mostly because it shares the tritone interval - F and B (Cb) - with the G7 chord.

But you want to be Mixolidian. That's fine, but Mixolydian doesn't have a dominant 7th chord containing a tritone. It doesn't even have a leading note a semitone below the tonic. It's questionable whether the term 'dominant' is applicable to the chord built on the 5th degree of the Mixolydian scale. If you're committed to Mixolydian, you need a different set of tricks to the sort of functional harmony that is all about dominant-tonic relationships.

When discussing harmony, people sometimes get hung up on the bVII chord (that's Bb major in the key of C). We notice that it sounds just fine to get home to C by Bb, F, C or even just Bb, C but agonize because it isn't diatonic. Well, in C Mixolydian, G7 or Db7 aren't 'diatonic'. You can use them, of course. But you'll lose the Mixolydian flavour. And I think we're still in a musical world where tonal can contain Mixolydian, but Mixolydian needs to maintain its identity.