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The flat five substitution is the cornerstone of modern jazz. It is a change that goes back to Art Tatum, and more famously to Charlie Parker and the early Bebop players. It’s largely because of this sort of harmonic change, that swing players, like Louis Armstrong and Benny Goodman, called Bebop “Chinese music.” (Curious that Chopin used the same sort of progressions in developing his chromatic harmony, but no-one considers him the least bit Asian. Goodman, who was a thoroughly well trained, and brilliant, classical clarinetist, should have known better.)
First, lets notice that the b5 is the tritone. It is the most dissonant of all the diatonic intervals. As a result, the key that is furthest away from the key of C would be the Key of Gb(or F#). Thus, it seems intuitively obvious that these keys would have little relationship to each other.
The dominant chord in C is G7. Its spelled G B D F. It’s perfectly acceptable from the rule of alterations to play G B Db and F instead. This is a G7b5.
Now, lets look at the dominant chord in the key of Gb, which is a Db7. It’s spelled Db, F, Ab, and Cb. If we do the same alteration on this chord, we get Db F Abb and Cb. Now, Abb is enharmonically the same as G; and Cb is enharmonically the same as B. Thus, in terms of pitches, the Db7b5 chord is Db, F, G, B.
Lo and behold, the two are the same. A 7b5 chord has the same pitches as the 7b5 chord built a tritone away. Thus, you can substitute the one for the other.
Conservative rule – For any dominant seventh chord, you can substitute a dominant seventh chord with the root a b5 away. Thus for a D7 chord, you can substitute and Ab7 chord.
Here is one of the most common ways that this is done. For a ii V I progression, say Dmin G7 C, you substitute the b5 of the G7, and arrive at Dmin Db7 C. This creates a nice chromatic descending baseline.
Aggressive Rule For any chord, you can substitute a chord of the same character with the root a flatted fifth away.
In the same progression, you could play instead Ab min G7 C, this time substituting the b5 of the minor chord instead of the major chord.
Why does this work? Remember that several of our rules are reciprocal. So if you have for example a minor chord, you can first do a ii/V substitution on the minor chord, making it a 7th chord, then a b5 substitution on the 7th chord, and then another ii/V substitution. thus Dmin becomes G7 which becomes Db7 which in turn becomes Ab minor. (Also remember that you can turn a major chord into a minor chord with maj/min substitution, and then do this same procedure again.)
On the Guitar, this type of substitution is very easy to apply. Assuming roots on the 6th 5th or 4th string, the b5 is either up one string and up one fret, or down one string and back one fret. With just a little bit of practice, you should be able to start improvising these substitutions in your playing. But remember, they can easily lead to clashes with the melody, so you still need to exercise some care, and adhere to the primary rule: If it sounds bad, it is bad.
I am attaching two examples to this lesson. One is simply another version of Amazing Grace, incorporating the b5 substitution, along with everything else that I have covered (Extensions, Maj/minor, ii/V subs). With these few rules, you can see how jazzy the simple progression can start to become.
The second example is a simple four bar Ragtime blues progression: C A7 D7 G7. This sequence of chords appears very frequently, and the example shows the simple progression, then with extensions, and then three ways you could use b5 substitutions.
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