The next chord we are going to manipulate is chord ii (D7), which was earlier a Dm7.
Just as in our previous lesson, we are going to identify the 5th of this chord: D F# (A) C.
Now we lower it by a half step, giving us Ab.
All we do now is use this note as the root of a new Dominant7 chord, leaving us with Ab7.
Again, both the 3rd and 7th of these chords are the same. They have just traded places. The 3rd of D7 becoming the 7th of Ab and so forth.
D7 - D F# A C
Ab7- Ab C Eb Gb (Gb is enharmonic to F#)
The progression is beginning to change drastically, however it still seems to have that strong gravitational pull towards C. This is the beauty of Tri-tone substitutions. It allows us to make huge changes to the sound of a chord progression. The fact that our 3rd and 7th tones are the same means that the harmonic 'pull' will still take us home comfortably, regardless of the root note of these new chords.
Meet me in the last part of this tutorial for our final substitution.