Considering the C major scale, its natural dominant chord is the G7 and we have been extrapolating chords that can be used to replace its function.
Commonly you will find references to a substitute of the V7 chord, abbreviated to “subV7” or “sub-7”, that is used to resolve to the tonic or to another target chord a half-step from above – i.e. Db7 – C∆. This chord movement could be considered a chromatic approach chord, but saying so it would limit its true potential.
We already saw this chord and how we can derive it from the diminished cycle, but there are other explanations from where it comes from. If you look at a G7 and a Db7, you will notice that both share the same tritone (F and Cb or B), and thus a perfect candidate for tritone substitution. And not only that but the 3rd and 7th of the G7 are the same notes as the 7th and the 3rd of the Db7:
Not only they share common notes (the tritone), but the chords themselves are separated by a tritone interval – a descending augmented fourth
There are only six tritones in the chromatic scale of the tempered tuning system, but if we have twelve chromatic notes then we have twelve possible dominant chords. This means that tritones are being shared by two dominant chords, and this is the case with these two dominant chords that are a tritone away from each other.
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