I’ve been trying to learn more about two chord substitutions—the tritone of the fifth chord and what I think is a iiib aug tritone substitution for the vi in a vi-ii-V-I sequence.
In the latter case, I’m thinking of the movement in older songs , such as “If I Had You” from I to iiib aug (?) to ii. It’s moving towards a brief stay on the V before resolving to I. In other words, since I don’t trust my own nomenclature, the notes, chords and progression in Bb are:


Bb-D-F (I)
Db-E-A (iiib aug) (?)
C-Eb-G (ii)
C-F-A (V in 2nd inversion, often)
Bb-D-F (I)

My questions, in case you’re wondering:
Is that Db-E-A chord an aug minor chord, or is there a better way to think of it? It appears to be an alternative to a milder sounding iiib dim (Db-E-G) in that same chord sequence.
Is there a good source of information about how these two substitutions are used in older songs? I can play many variations, and I’m hearing many possibilities. I understand that one can use a triton sub just about anywhere, but my focus is just on this older sound. Any books or sites that spend some time on this?

Hate to answer myself, but it turns out that the Db-E-A chord is probably an A major chord with the third in the bass and the root on top. So the progression is taking a chromatic step down in the chords, but moving up in the bass, and using the first inversion on that A chord, so the root on top gives it a sense of moving up instead of down.

Still a little odd, though, since the vii is not a passing chord. The vi gets left out entirely, so the sequence becomes I - vii - ii - V - I.

There is a really old book (70s or early 80s, I think) called "How to Create Jazz Chord Progressions" by Chuck Marhonic (I always thought that this guy either has the perfect name for his profession, or at least it is an anagram).

I found it to be slightly different than it's title...it is really a primer on jazz chord progressions, concentrating on ii-V7s and tritone substitutions.

You may already know this, but the general rule (in a jazz context) is that you can pretty much substitute, for any dominant chord, a dominant chord that is a tritone away from the root. The reason why is that both chords share the exact same notes as the third and the seventh. One place where this may not work as well is if a melody note is the root or fifth of the original chord. That is, for a tritone substitution, the 3rd/7th stay the same, and you are altering the root/5th (so if the melody note is a root or 5th, then altering it may clash with the melody).

This was all really clear to me after I worked through Marhonic's book.

Like most jazz instruction books, you can always find them at Jamey Aebersold's site (www.jazzbooks.com).

Guy

On the topic of books ... I found the following to be really good at discussing in depth some of the harmonic devices used in jazz compositions.

"The Harmonic Language of Jazz Standards"

http://www.outsideshore.com/school/music/harmonic/

Thanks for those suggestions. I'll look into those books. I must say that this progression is giving me a lot of pleasure.

It's a strange thing. The original progression that I changed was Bb-Db dim-C minor-F-Bb, which is a fairly conventional turn around, according to what I was told in the KVR theory forum, where I also posted this question. But changing the flatted 5th to a sharp 5th changes it to a new chord while keeping the same general sense of motion, opening up a lot of new harmonic possibilities and scale notes. I'm sensing that I'm scratching the surface, too.