Substitute Dominant chords, sometimes referred to as tritone substitutes, are some of my favorite chords in jazz. Some songs have them already in the chord progression, other times the player will play the substitute dominant instead of the regular or secondary dominant Additionally, players will precede chords with substitute dominant chords to add more chords to a progression (this is one of the easiest and one of my favorite ways to sound more "sophisticated").
So what is a substitute dominant chord?
Well, it's a dominant chord an augmented fourth, aka tritone, away from the dominant chord it's substituting for.
Now the term "substitute dominant chords" is a real mouthful, so they are often called subV's ("sub five's").
So in the key of C, the primary dominant is the V7 chord, G7, and the subV for G7 is Db7. The subV for the V7ofII, A7, is an Eb7. So a I VI7 II-7 V7 progression with substitute dominants would be C Eb7 D-7 Db7.
Notice that the SubV for G is Db and likewise the subV for Db would be G.
Dominant V chords can have a variety of chord tensions, but subV's always have the same: 9, #11, 13.
A common trick to make this easier is to play a major triad a whole step above the dominant chord. So on the subV of F#7, which is C7, you can play a D major triad on top of the regular C, E, G, and Bb. Note also that the tensions of 9, #11, and 13 all sound great and very jazzy.
How do you reharmonize chord progressions with subV's?
As I said above, sometimes they are part of a chord progression naturally, but if you want to change a chord progression, here is how:
1)Replacing dominants with their substitutes. It's pretty self-explanatory, here is a jazz blues chord progression with substitutes:
C7 / F7 / C7/ G-7 Gb7
F7 / F#dim7 / C7 / E-7 Eb7
D-7 / Db7 / C7 Eb7 / D-7 Db7
Notice that I only substituted the dominants that were a V7 of something (including the primary dominant, G7).
2)Preceding chords with subV's. Just add a dominant chord with 9, #11, and 13 a half step above the chord it's proceeding. This creates a lot of chords and can be sometimes done to excess. Here's the same blues progression with subV's added but not replacing the existing dominants:
C7 Gb7 / F7 Db7 / C7 Ab7 / G-7 C7
F7 / F#dim7 Db7 / C7 F7 / E-7 A7
D-7 Ab7 / G7 Db7 / C7 Bb7 A7 Eb7 / D-7 Ab7 G7 Db7
Note the last two bars have a chord on every beat with every possible subV. Usually, this can be a little crazy but I like it at the end of the progression to build tension and impress listeners. You can even play subV's of subV's, but it will significantly take it out of key.
Also, there are a few chords that shouldn't have a subV preceding them, such as diminished chords and minor 7 b5 chords (instead play the subV of the dominant that follows the -7b5, but play the subV before or instead of the -7b5).
SubV's sound great and I love playing just an occasional subV now and then, or playing a ton of them, both while comping and soloing.
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