A dominant seventh chord resolving down a perfect fifth is the most powerful harmonic
gesture in music.
We all know the sound.
The ultimate resolution.
Of course, it's also an extremely common and straight-forward sound.
I'm gonna show you how to make this idea much more interesting.
But first, we need to take a closer look at the chord itself.
The dominant seventh chord occurs naturally on the fifth degree of a major scale.
Structurally, it's a major triad plus a minor seventh.
Now, this structure contains a very special interval.
There are six semitones between the major third and the minor seventh of the chord.
This is a powerfully dissonant interval, known as the tritone.
Think of the tritone as the backbone of the dominant seventh chord.
This inherent dissonance is one of the reasons these chords usually sound like they really
want to resolve, specifically to the chord a perfect fifth below them.
The notes in G7's tritone, B and F, resolve perfectly to the C and the E in C major, the
chord a perfect fifth below.
Okay, here's where it gets interesting.
Since tritones are exactly half of an octave, they are inversionally symmetrical.
In other words, when you invert a tritone, you end up with the same tritone.
If you think about it, this means any given tritone really exists in two different dominant
seventh chords, and those chords would have to be a tritone away from each other.
And, since the tritone is such a big part of the function of these chords, we should
have two different dominant seventh chords that both want to resolve to the same place.
Welcome to tritone substitution.
Okay, let's see: the dominant seventh chord a tritone away from G7 would be Db7.
We just need to re-spell our B as a Cb and we can see they do indeed share the same tritone,
only inverted.
What was originally the major third of G7 became the minor seventh of Db7, and vice
versa.
Okay, don't worry if you didn't quite follow all that logic; here's the bottom line: If
we want to target a certain chord, for example, C, we can do so using the dominant seventh
chord a perfect fifth above it, which is G7.
Or, we can use the dominant seventh chord a half-step above it, Db7.
In this case, Db7 would be a "substitute dominant chord," or "tritone sub."
It does basically the same job as its more common counterpart, but in a more colorful
way.
By the way, we use these arrows to indicate dominant resolutions.
A solid arrow for a dominant seventh chord resolving down a perfect fifth, and a dotted
arrow for a dominant seventh chord resolving down a half-step.
So, we can use a solid arrow or a dotted arrow to lead into almost any chord.
Here's a progression that uses both types of dominant resolutions.
Now that we have the basic concept down, it's time to take it to the next level.
It's very common and effective for the II-7 chord to lead into the V7 chord.
In this situation, II-7 is what's called a "related II chord."
That's a minor seventh chord leading down a perfect fifth into a dominant seventh chord.
We analyze this relationship with a bracket.
So, we have a bracket leading into an arrow, and it sounds like this.
But, related II chords don't have to be the actual II chord in the key.
We can use this bracket relationship to setup any dominant seventh chord.
Let's see what happens when we add a related II chord to our subV7.
The minor seventh chord a perfect fifth above Db7 would be Ab-7, a very unexpected chord
in the key of C Major.
So this chord will sound puzzling at first, but once you hear where it's going, it works
really well.
Now, let's get weird.
What if we take Db7's related II chord, but swap G7 back in?
Now we have a minor seventh chord leading down a half step into a dominant seventh chord.
This is called a substitute related II chord, indicated by a dotted bracket.
And how about a dotted bracket with a dotted arrow?
We can even start chaining together and combining these brackets and arrows in all sorts of
wild ways.
The possibilities are endless.
Here's a little example.
So, substitute dominants and related II's can give you some pretty far-out sounds.
But, since these chords have such clear function to them, they tend to sound a little more
palatable than you might expect, given the amount of chromaticism involved.
This is one of my go-to techniques if I'm looking for something interesting, but not
too difficult to listen to.
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