Flat Five Substitution

The flat five substitution is a chord that you can use in place of a dominant chord yet the chord quality is preserved. This is mostly helpful and very well suited for the twelve bar blues because it uses dominant seventh chords frequently.

When a progression has a dominant chord, you have a chance to substitute a chord whose root is a diminished fifth above the original chord. Suppose you are playing an E7 chord. The notes are: E-G#-B-D. The fifth in this chord is B. Hence flat fifth would be Bb. So, the substitute would be Bb7 chord and it will sound nice and perfectly fit to the progression.

Let us find out why it fits so well there. The notes in this Bb7 chord are: Bb-D-F-G#. You may notice that the third  and seventh (G# and D) are interchanged in the flat five substituted chord. It means that we still have the most important chord tones preserved in the substitution but they have interchanged their degrees. Hence the substituted chord has the same quality and yet a different colour which sounds awesome. This explains why the flat five substitution works.

Another great thing about the substitution process is that you do not always need to substitute a dominant seventh chord for a dominant seventh chord. You can substitute any dominant chord for that. for example you can substitute Bb13 for an E9 chord.  For even more possibilities, one can substitute a dominant chord with a flat five note added. For example you can substitute an E13b5 for an E7 chord or you can substitute a Bb13b5 for an E7 chord. You can do this because Bb and E are tritones for each other. in other words, they are a diminished fifth from each other.