Expanding on the previous page, take a look at the variation below.
On the previous page we used "dominant approach" and related ii-7 chord to superimpose a ii-7 / V7 / I in C over the original harmony.
In this example, I have replaced the superimposed G7 harmony with a b5 substitution
Any dominant 7th chord can be substituted with another dom7 chord whose root lies a b5 away.
The b5 sub for G7 is Db7:
The reason this substitution works is because both chords share common tones. In fact, the basis for all chord substitution is the sharing of common tones (usually two or more). The common tones shared between dom7 chords whose roots lie a b5 apart form a structure called a tritone
All dominant 7th chords have an interval structure within them which is called a tritone
. A tritone is the distance between the 3rd and the b7 of a dom7 chord, and it spans the interval of a b5. You will notice that even if you invert it, it remains a b5. This is because it splits the octave exactly in half. It gets it's name from the fact that it is comprised of three steps, or whole tones...hence the name, "tritone". Because of it's omnimous sound and mysterious properties, this interval was banned from music during medival times and was commonly referred to as "The Devil's Interval". It found it's way into many hard rock and heavy metal tunes (bands like Black Sabbath), probably for the same reasons. They also have a very recognizable, "diagonal" shape on the fretboard. Have a listen:
If you examine a dom7 chord and it's b5 substitute closely, you will notice that they share the same tritone. Using our G7/Db7 as an example, note that the 3rd (B) and b7 (F) of G7 form a tritone:
The 3rd (F) and b7 (B, enharmonically Cb) of Db7 also form the same exact tritone:
Notice how the interval inverts itself? The 3rd of G7 (B) now becomes the b7 (Cb) of Db7, and the b7 (F) of G becomes the 3rd (F) of Db7, but the interval remains a b5 tritone. This is why the b5 substitution works...cool, huh?
There's more! What happens between the roots and the 5ths of each chord? We already established that the roots of each chord are a b5 away (another tritone!). What is a Db note compared to a G7? An altered tension, specifically the b5. What's a G note compared to a Db7? Again, the altered tension b5. The 5th of Db is Ab. What is Ab compared to G7? Another altered tension...this time it's the b9. The 5th of G is D. What is D compared to a Db7? Again, the b9! Cool! It seems that a straight b5 substitution will function
as a dom7(b5,b9)....an altered dominant chord. Db7 in place of G7 functions as G7(b5,b9), and G7 substituted for a Db7 functions as a Db7(b5,b9). So you see, even a b5 substitution's function
remains the same when inverted. Spooky, huh?
The b5 substitution also has very useful voice-leading implications. We can now alter the voices of our G7 chord so that every voice moves by a half step when it resolves to C (or, in our case, C7). This creates extremely smooth, chromatic voice-leading results, especially in a ii/V/I progression. Instead of this:
We can get this:
Very nice! Check out this concept as applied to our arrangement. On the 2nd beat of measure two, we now have a reharmonization that utilizes dominant approach, a related ii-7 chord, a b5 substitution, and improved voice-leading.
To generate even a little more movement in the bassline and middle voice, I have revoiced measure two's final C7 chord with a C9/G: