Flat five substitution theory lesson

Today’s lesson will delve into the world of flat five substitution, what it is, and how you can use it in your playing and compositions, so let’s dive right in!

Contents

What is flat five substitution?

Flat five substitution is a type of chord substitution, in which a functioning dominant seventh chord can be replaced by forming another dominant seventh chord, upon the diminished fifth from the original dominant seventh chords root. It’s a mouth full I know, but the process is really quite simple, so lets go through the example below, step by step.

In the example below, you will see a simple chord progression with two highlighted areas in red, the first is highlighting the V chord which is G⁷ as we are in the key of C major, and the second is highlighting our flat five substitute which is D♭⁷ because D♭ is the diminished fifth of G⁷, and we have simply built a dominant seventh chord upon that note. Let's look at the relationship between these two chords to see why flat five substitution works.

The notes of the G⁷ chord are: G-B-D-F.

The notes of the D♭⁷ chord are: D♭-F-A♭-C♭.

So why does this work? Let's take a closer look, you know that a triads' most important note is the third right? The third determines if the chord will be major or minor, and the third in G⁷ is the note B. In order for G⁷ to be dominant, it must also have a minor seventh, which is the note F and is the other important tone in our chord. Now looking at our D♭⁷, you will see that the third is the note F, and the seventh is the note C♭ which is enharmonic to the note B, so they have switched roles but they are none-the-less, the third and seventh degrees of our original G⁷ chord. This is why flat five substitution works because the crucial notes of the V chord are also in the D♭⁷, which enables us to resolve to the root in the same manner as G⁷ does.

So the end result of flat five substitution is an altered bass line that descends from D, to D♭, and then to the root C (in our example), which provides a nice voice lead and a great alternative to the V-I move. Flat five substitution will work with any functioning seventh chord, in other words a dominant seventh chord that resolves to the root. Flat five substitution chords are analyzed as ♭II⁷ and can be applied to the V⁷ of any minor key as well.

Summary

Well I hope you enjoyed this lesson on flat five subbing, it's a lot of fun and adds a fantastic new sound for your compositions.

Cheers & enjoy!