Tritone Substitution - What exactly is a b5 sub?

Harmony - The Tritone Sub

This lesson has been revised and published in THE INFINITE GUITAR. Info >>>

The tritone substitution, or sometimes know as the b5 sub, is a very common substitution. It does sometimes cause a little confusion so this lesson will deal with how to play and use it and what it is theoretically.

What Is a Tritone Substitution?

What exactly is a tritone substitution anyways and when do you use it? The answer is simple, when you have a dominant chord that resolves to a tonic chord (V - I) you can substitute another dominant chord a tritone (b5th) from the root of the original dominant chord. In other words, if your chart says play this: Dmin7 - G7 - Cmaj7, you can play this: Dmin7 - Db7 - Cmaj7:

with the bV sub >

That is the rule, the problem is that it is a little too simple and at some point in your (musical) life, you want to know why it works and how to expand on it.

A word on substitutions - First let me clear up some things about substitutions, when you play one, assume that the bassist doesn't know you are going to do it and you don't need to mention it. This means when you play your substitution, he will not have a clue and play the bass note of the original chord on the chart. This is a good thing. If he played to bass note of your sub, it wouldn't be a sub, it would be a reharminization. You see, what you are really doing by playing a sub, is creating a slash chord in a way (a chord over a bass note). Before we get to the tritone sub, let's take a look at some other common substitutions. These are two standard subs for major chords. The first is a min7 chord a min 3rd below the original chord, and the second, a min7th chord a third above. In other words, if your chart tells you to play a Cmaj7, you can play an Amin7 chord or an Emin7 chord. Look at the example below:

Our simple ii - V - I in C:

Min7 chord down a 3rd - Instead of playing the "I" chord as a Cmaj7 chord, you are going to play an Amin7 chord in its place:

Remember, the bassist is clueless about your plan so while he plays the root, C in this case, you are going to play an Amin7 chord and the result will be this: Amin7/C. Look at the analysis below and you'll discover that an Amin7/C chord is really just a plain old C6 chord (1, 3, 5, 6):

Min7 chord up a 3rd - Another common sub for a "I" chord is a min7 chord a 3rd above. In this case an Emin7 chord rather than the Cmaj7 chord:

And now take a look at the analysis of what chord we really get when we substitute an Emin7 chord for our Cmaj7 chord. Don't forget, the bass player is still going to play a C bass note. You can see that an Emin7/C slash chord is the same as a Cmaj9 chord (1, 3, 5, 7, 9):

The previous two examples are both very common substitutions, I used them as examples as they are simple and easy to understand. Substitutions are simply tools to create chords with extensions. What other type of substitutions could you use as replacements for a simple "I" chord? Try these: Gmaj7 or D.

The Tritone (b5) sub

The b5 sub - In order to see why the tritone sub works, we'll need to analyze it the same way we did our other two examples. Remember, your bassist is going to play a G bass because he doesn't know of your plans, and you, the sub, which in this case will be a Db7 chord. So what we end up with is a Db7/G slash chord. Take a look at the Db7 chord and then look what happens when we superimpose it over the G bass note the bassist is playing:

now, superimposed >>

Analysis: Db7/G = G7(b5,b9)

Expanding on this concept - This whole thing got me thinking one day. I figured that if a dominant 7th chord a tritone away from the root of a "V" chord would yield me a nice 7(b5,b9) chord, what would one of the bigger dominant chords yield. First I replaced the Db7 chord with a Db9th chord and look what I got:

now, superimposed >>

Analysis: Db9/G = G7(b5,#5,b9)

Next, a Db13th chord. The altered mother lode! This chord superimposed over a G bass note gives you one whole complete altered chord with all four alterations!

now, superimposed >>

Analysis: Db13/G = G7(b5,#5,b9,#9)

Looking from a different angle - Just so you can see things a little clearer, look at the G7(#5,b9) chord and then compare it to the Db9 chord next to it. Pay special attention to the top four strings and it becomes quite simple to see that they are pretty much the same chord:

Things to consider - What would happen if you tried a Db7(#9) chord as a b5 sub? What chord would this give you and what scale would it be coming from?

Summery

The tritone sub is basically a simple way to create an altered chord. In all reality, they are really the exact same chord coming from the same scale: G7(alt) = G altered scale (Ab melodic minor). Db7 = Db lydian dominant scale (Ab melodic minor). You can use the substitution principles I've discussed to make lines also:

Now that you know how substitutions work and what they are, try to figure out some other ones that might be of use. Until next time....