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In this progression we have used a tritone substitution (bV sub) to replace
the D9 chord. We can do this because the Ab7 chord contains the same two
notes that determine the quality of the D9 chord.

Let’s analyze that:

D9 = D F# A C
Ab7 = Ab C Eb Gb(F#)

We can see that the major 3rd(F#) and flat 7 © from the D9 chord are
inverted in the Ab7 chord (C is the major 3rd and Gb(F#) is the Flat 7).
Those two tones are the notes that give a dominant 7th chord it’s quality.

Why do we do this? Well, it’s because it gives us a chromatic bass line from
the minor 7 chord down to the major 7 chord, a smooth transition between
chords, and sounds wonderful.



 





 






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