This video will talk about the concept of Tri-Tone Substitution. This phrase - “tri-tone substitution” is very much a 20th century term coming out of the jazz and popular music world but the concept itself is centuries old and forms the basis for the traditional Augmented sixth chord.
The Tri-tone interval occurs naturally in a western tonality between scale degrees 4 (Fa) and 7 (Te) you can find a tri-tone here in all twelve keys in both major and minor. In C major these are the notes ‘F’ and ‘B’. You will also recognize these notes as chord degrees 3 and 7 of the Dominant chord. Again: remember this works in all 12 keys.
As we understand already from traditional harmony, this tri-tone interval likes to resolve inward or outward by 1/2step depending on inversion. Here, I am playing G7-CM with the tri-tone resolving inward. Here, I am playing G7-CM with the TT resolving outward. This tension and release built on the resolution (or not) of the TT is the bases for traditional harmony.
The Tri-tone substitution concept takes advantage of a naturally occurring musical phenomenon where the tri-tone splits the octave exactly in half. Because of this, the Tri-tone (or Aug4th/Dimin5th) is completely symmetrical.
Because the TT is symmetrical with only 6 versions and is found in all 12 dominant chords. This means that every dominant chord shares a tri-tone with another dominant chord. Or you could think of this as every TT has 2 dominant chords that could be built of it.
If I take C-F#(Gb) I could create a Ab 7 chord by thinking of the C as the 3rd and the Gb as the 7th. If I flip this around, I could create a D7 chord by thinking of the F# as the third and the C as the 7th.
I recommend pausing the video here and playing with this concept yourself at the piano before going on.
Now that we understand how each Dominant Chord has a partner that shares the same Dominant chord, Let’s look at how to use this concept in music. First, notice that the internal tri-tones (3rd-7th) in any dominant chord switch places. Second, notice that the roots of these two dominant chords are also a TT away.
Here is a G7 chord with our tri-tone of B and F. G7’s traditional resolution is to go to C major - and in this chord voicing, the TT resolves inwards by step. However, because of this symmetrical interval, we could also resolve this TT in this chord voicing outward by step giving us the notes A# and F#. This is now F# Major… A TT away from G7’s traditional resolution of C major. Also notice that G7 is a half-step above the root of F#.
Here are the pieces to keep in your mind with TT Substitution:
Dominant chords can resolve (1) traditionally with root motion of 4th/5ths around the circle or (2) downwards by step.
(above info on screen)
Another way to think about the same concept is that any given chord can have two different dominant chords that could go lead to it.
Here are a couple more examples.
If I am doing a ii-V-I in C major: d- G7 CM
I could use this concept to substitute my Dominant Chord for Db7 and create this. D- Db7- C. Both of these function normally in western harmony. Composers from the great American songbook, such as George Gershwin, used these substitutions all the time.
Here is another example where I will swap the resolution:
d- G7 as if I am in C major, but I will use my TT Sub rules to swap my resolution to F# major instead.
I recommend playing with this concept at the piano in all keys. Once you get the hang of it, this is a very simple concept that can open the doors to many more harmonic options for you and to help partially understand how composers have used the augmented sixth chord concept in the past.