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If you have played a lot of my arrangements such as last month’s “I Need Thee Every Hour,” you have seen me do this kind of thing quite a bit:
Note what happens in bar 31. You guys know by now that I automatically change almost every V-I progression in music to a ii-V-I. However, this is not a ii-V-I progression. It starts on a ii (Am7) and ends on I (G) but the chord in the middle is off the beaten path.
If you study it a second, you should recognize it as an AbM7 (Ab, C,Eb, and G). If we were going to refer to that chord by number, we would call it bII. We are in the key of G and A natural is the second note in a G major scale. If A natural is II, then Ab is bII. The G in the chord makes it bIIM7.
Therefore, here is the progression you see there:
Let me give you a very simple rule. Melody permitting, you can always substitute ii-V-I for V-I and you can always substitute ii-bII-I for ii-V-I. The bII can be a major 7th or it can be a dominant chord. All in all, that gives you several different options to spruce up a boring V-I cadence into something a bit more interesting.
Here is the theory behind it. In the world of functional harmony, chords that are a tritone apart are often considered equal and can substitute for each other. A tritone is a half octave or an augmented 4th (a perfect fourth + one half step). The tritone for a V chord is bII. Therefore, ii-V-I can be changed to ii-bII-I without any issue.
Usually, these tritone substitutions involve dominant chords and maybe at some point, I will explain why from a theory perspective, a bII7 functions almost exactly like a V7. However, it also works in this situation where I am using a bIIM7 rather than dominant. Sometimes, the melody makes that decision for you. In this example, I could not have used bII7 because I would have ended up with a clash between the melody note (the major 7th) and the minor 7th that is found in a dominant chord.
Note how the progression I chose here created a very different bass line that moves in steps rather than leaps of 5ths as happens in ii-V-I progressions. That creates a smooth sound that is very distinctively different. I would not call it superior or inferior. I would just call it different.
Last, notice that this progression creates parallel fifths in the left hand between the last two beats of bar 31 and bar 32. This is often a red flag to traditional music theory folks but is intentional in this context and just fine.