Scott, this was a great comment and you are mostly correct!

Yes, the easy way to do a flat-five substitution is to lower the V7 chord a half step to the bV7. This will resolve very nicely to the tonic (aka. I chord). In fact, all four notes are a half step from the IMaj7 chord tones for a very strong resolution!

The II7-I comment is not exactly correct, but you’re in the ballpark. The flat II7 (bII7) shares 3 out of 4 notes with the bV7 and the 4th note resolves to the IMaj7 just as well. It can be used in a similar way, but it is not the same substitution.

The flat five is often used in a ii-V-I progression. You can get a chromatic line by playing iim7-bV7-I. Let’s look at the notes from the key of C. iim7 (D,F,A,C), bV7 (Gb,Bb,Db,F), I (C,E,G,B). Notice how every note in the iim7 is within a half step of every note in the bV7. The same holds true with the bV7 to the I. The cool thing is that you can play this in a very tight space on the guitar neck.

It sounds very complicated, but chances are you’ve already used it, or at least sounded it out. Essentially what you are doing is a II7-I progression. Here is how you get to it, the way *I* understand it anyway.

Let’s say you’re playing in Amaj. Take a basic I-IV-V progression. In “A” the dominant is “Emaj”. That makes E-G-B. You take the fifth of that cord and flatten it. That would be Bb. Now all you have to do is make that a Dominant 7 and resolve back to the tonic.

I hope that is right and that it helps.

Toby, I am working on an update to this lesson that includes example progressions. It isn’t ready yet. In the meantime, use the links in my sidebar to follow me on facebook and twitter. I will announce the updates there when they’re available.