The flat five substitution is a chord that you can use as a substitute for any dominant chord. It applies very nicely to the 12 Bar Blues, because of the use of Dominant 7th chords. Enough chit chat, lets get to the facts.
When you play a dominant chord there will always be a chord that you can substitute for it and still hace it sound good. That substitute chord is a diminished 5th above the the original chord. So if you are playing an E7, the substituted note would be Bb7. It is called a Flat-Five Substitute because if you are in the key of E the V note is B, and the bV is Bb hence the name Flat-Five.
The Flat-Five chord substitutes so nicely because of the notes which are contained in the original chord and the Flat-Five chord. Let’s look at the notes in each chord.
If you will notice, the 3 note and the b7 note are the same notes but switched around the chords. Remember that Ab = G#. That is why the chords can be substituted for each other.
The reason that the notes are the same is because the interval between the 3 note and the b7 note is a diminished 5th (also called the tritone). The neat thing about the tritone is that it divides the root note and octave in half. In other words, It is the same distance from the root note as it is from the root note’s octave. This is why you can get the same notes for both chords. Because all dominant chords (7th, 9th, or 13th) must contain the 3 and b7 notes, this subtitution process works every time.
Another great thing about this substitution process is that you do not have to substitute a Dominant 7th chord for a Dominant 7th chord. You can substitute any dominant chord for any other dominant chord. For example you can substitute a Bb13 for a E9 chord.
For even more possibilities, you can substitute a dominant chord with a flat-five note added. For example, you can substitute an E13b5 for a E7, or you can substitute a Bb13b5 for an E7. You can do this because Bb and E are tritones of each other. In other words, they are a diminished 5th from each other so the note that is a b5 of Bb is E, and the note that is a b5 of E is Bb. If you did not know it already, this is the reason that the blues scale has a #4/b5 note added to it.
This lesson should really broaden your playing vocabulary. It will really help you if you are a jazz or blues player who uses a lot of Dominant chords.