A ‘tritone substitution’ is a popular trick for making your music sound more sophisticated. I’ll show you a couple of videos with lots of examples. But since I’m mathematician, let me start with the bare-bones basics.
The fifth note of the major scale is called the dominant. In the key of C it’s G.
A dominant seventh chord is a 4-note chord where you start at the dominant and go up the major scale skipping every other note. In the key of C it’s the notes in boldface here:
G A B C D E F
Below I’ve drawn blue edges from G to B, from B to D, and from D to F:
Any dominant seventh chord has two notes that are opposite each other—shown in orange here. We say they’re a tritone apart.
A tritone is very dissonant, so the dominant seventh chord really wants to ‘resolve’ to something more sweet. This tendency is a major driving force in classical music and jazz! There’s a lot more to say about it.
But never mind. What if we take my picture and rotate it 180 degrees? Then we get a new chord! This trick is called a tritone substitution.
Let’s look at it:
Here you see our original dominant seventh chord (with notes connected by blue edges):
G B D F
and its tritone substitution (with notes connected by red edges):
C♯ F G♯ B
Each note in the tritone substitution is a tritone higher than the corresponding note in the dominant seventh chord—so it’s the point on the opposite side of the circle.
But two notes in the dominant seventh were directly opposite each other to begin with: B and F. So these two notes are also in the tritone substitution!
This makes the tritone substitution sound a lot like the original dominant seventh chord.
See the square of notes? Three are in the original dominant seventh, and three are in the tritone substitution. Musicians like such squares of notes, even though they’re quite dissonant. They’re called diminished seventh chords. But let me stop here instead of blasting you with too much information.
Let’s actually listen to some tritone substitutions:
Here David Bennett plays a bunch of songs that use tritone substitutions. I’m very glad to learn that the slippery, suave sound at a certain point of The Girl from Ipanema comes from using a tritone substitution. He also plays a version without the tritone substitution, and you can hear how boring it is by comparison!
Music theorists tend to bombard their audience with more information than nonexperts can swallow in one sitting. David Bennett could have made a nice easy video full of examples where musicians apply a tritone substitution to a dominant seventh chord in its most natural position, where it starts at the 5th note in the major scale. But you can also take a dominant seventh chord and move it up or down, getting a chord called a secondary dominant, and then do a tritone substitution to that. And Bennett feels compelled to show us all the possibilities, and how they get used in fairly well-known tunes.
If you start getting overwhelmed, don’t feel bad. Let me show you another explanation of tritone substitutions:
I really love the friendly, laid-back yet analytical style of Michael Keithson. He’s great at explaining how you could have invented harmony theory on your own starting from a few basic principles. He’s like the music theory teacher I always wish I had.
His pose above is a parody of the sneaky punch that a tritone substitution can pack. Speaking of which, if you want to sound like a hipster, you say tritone sub. Don’t mix this up with the submarine used by the Greek sea god born of Poseidon and Aphrodite: that’s the ‘Triton sub’.
In this video Keithson explains tritone subs. He starts out basic. Then he gets into all the ways you can use a tritone sub. You may want to quit at some point when it gets too complicated. I think it’s better to just zone out and listen to his piano playing. Even if you don’t follow all his words, you’ll get a sense of what tritone subs can do!
Understanding tritone subs well requires understanding dominant chords, and Keithson has a good lesson on those, too: