Tritone substitution is a a kind of chord substitution that can give added musical interest to a progression and/or enhance melodic lines. Using this concept tends to give your progressions a jazzy feel and it can be a very nice change to a stale chord progression in any genre you can make it fit in. It's used extensively in jazz, but I've seen it used in progressive music too, and sometimes even in rock and pop music! And personally, this is one of my favorite chord substitutions.
There are some prerequisites in order to really be able to understand and use this concept. If you don't have a working knowledge of any of these concepts, there are some great places that a google search will take you, and I'm also totally willing to help explain them if you PM me.
1) You should know some music theory, most importantly your scales, scale degrees and what chords are normally built off them, and your intervals (perfect 5th, diminished 5th, minor 3rd, etc). With basic music theory, you could push through this lesson, probably with some help, but if you're closer to an intermediate amount of music theory knownledge it'll really help you here.
2) You should also know how to construct/play major, major 7, dominant 7, minor, and minor 7 chords.
3) A good working knowledge of the notes on the fretboard is also very, very helpful here, so you don't have to spend a lot of time counting up the fretboard to make sure you're really on the right fret.
Alright, now on to the good stuff!
One of the more interesting things about the tritone interval is that if you invert it, it is still the same interval; the tritone interval divides the chromatic scale in to equal parts. Let me show you what I mean:
Let's look at an interval other than a tritone, like a perfect fifth between the notes G and D:
And here's how it looks in terms of the chromatic scale - all the notes that exist in our music:
A A#/Bb B C C#/Db D
D#/Eb E F F#/Gb G
G#/Ab A A#/Bb B C C#/Db D
D#/Eb E F F#/Gb G
You may or may not remember, but if you take a perfect fifth interval (like G to D) and invert it (so now it's D to G) it becomes a perfect fourth interval. And now look at the chromatic scale above; you'll notice that there are six notes between G and D, but only four notes between D and G.
So when you invert an interval, it becomes a different interval. This is true for every interval except one: The tritone.
Now let's see what the tritone interval between B and F looks like on the chromatic scale:
A A#/Bb B
C C#/Db D D#/Eb E F
F#/Gb G G#/Ab A A#/Bb B
C C#/Db D D#/Eb E F
F#/Gb G G#/Ab
Notice that there are exactly five notes between B and F AND between F and B; It doesn't matter which note you start on. The tritone interval is the ONLY interval that divides the chromatic scale into equal parts like this.
Now, you also may or may not remember that the characteristic sound of a dominant 7 chord comes from the tritone interval that exists in the chord, which is between the major 3rd and minor 7th.
Here, you can see that in a G7 chord, the tritone interval exists between the notes B and F (the major 3rd and the minor 7th, respectively).
*Now really pay attention to this part, it can get a little confusing and/or complicated*
Remember how we just saw that if you invert a tritone interval, you get another tritone interval with the same notes, just a different order
? How no matter how you look at it, the notes are the same distance apart?
If G7 has the tritone interval from B to F, it's major 3rd to it's minor 7th, and since B and F are the same distance apart whether you start from B or from F, isn't there another dominant chord where the major 3rd is F and the minor 7th is B, instead of the other way around?
In fact, there is! Have a look:
Db7, has the notes Db, F, Ab, and Cb(which is the same as B). Notice that the tritone interval, the major 3rd to the minor 7th, is F to Cb, which is enharmonically the same as F to B (Cb and B are the same note), and is the same tritone as the one in G7! Don't forget that it's only the same because the tritone interval splits the chromatic scale into equal parts, and therefore, is it's own inversion!
Another cool thing: G7 and Db7 have the same tritone, but did you notice that their roots, G and Db, are also a tritone apart?
*Note: Db7 is also the same thing as C#7 (C#, E#(same as F), G#, and B) but in my experience I've seen tritone substitutions spelled using the diminished fifth (b5) definition of a tritone, not the augmented fourth (#4) definition.*
Tritone substitution says that because the tritone is a big defining quality of dominant 7 chords, you can substitue a dominant 7 chord for the other dominant 7 chord that has the same tritone. Let's look at that now.
Now we have two chords that have the same tritone interval: G7 and Db7. Hopefully you also remember that the primary function of a dominant chord is to resolve to the chord a perfect fifth below (or perfect fourth above), like this example in the key of C, where G7 resolves to C:
And now, in context, tritone substitution is when you take that dominant 7 chord, G7 in this case, and substitute it with the dominant 7 chord whose root is a tritone away from the original chord's root, which is Db (the tritone of G is Db, and vice versa). So instead of playing what is depicted above, you could use Db7 instead of G7, and have this:
But let's try this with something a little more musical: a standard ii-V-I progression with 7th chords!
Here is a regular ii-V-I in the key of C:
Sounds alright? Yeah, it's THE standard progression in jazz music, haha. But check this out!
Now THAT'S cool!
You'll notice that the first progression is notated as "ii-V-I", but if you look at the picture for the tritone substitution progression, you'll see that the new dominant 7 chord, Db7, is notated as "subV7/I". You read this as "sub five of one". This chord is the substitute V chord relative to the I chord. That just means that instead of playing the regular dominant 7 chord, which is built off of the V degree of the scale, you play the chord you get when you do a tritone substition, hence "subV/I". You'll also see this notated as "subV", which is pronounced, as you might have guessed, "sub five". You can also hear these substituted chords referred to as "substitute dominant" chords.
You can also use tritone substitution in a minor key context, like this:
-The tritone interval is the ONLY interval the divides the chromatic scale into equal parts, and therefore is it's own inversion
-There are always two dominant 7 chords that have the same tritone, and those chords' roots are also a tritone apart
-The tritone is an integral part of a dominant chord's characteristic quality.
-When you have a dominant 7 chord that resolves down a perfect fifth or up a perfect fourth (like they normally do) you can substitute that dominant 7 chord with the dominant 7 chord who's root is a tritone away from the original chord's root. This is called tritone substitution.
-The new chord you get when you perform a tritone substitution can be notated as "subV/I" or "subV" and is also referred to as a "substitute dominant" chord.