Today we’re going to look at some ‘Tritone Substitutions’. So what is a Tritone and what is a Substitution?
A tritone is an interval, also known as an Augmented Fourth or a Diminished Fifth. It is an interval of 6 semitone’s, or 3 whole-tones. A tritone above G would be Db, so we can play a tritone by playing the open G string followed by the Db on the 6th fret of the same string.
What we mean by the term ‘substitution’ is that we are substituting, or swapping, one chord for another. This is a good way of spicing up your progressions by swapping in chords borrowed from a different key. You can also alter chords in the key by adding notes from outside of it. This can be a great way to add colour to a bland progression.
Now that we know what a tritone is and what is meant by ‘substitution’ we can look at combining them. Tritone Substitution is usually applied to a Dominant 7 chord. It can be used to create a descending bass line in a II – V – I progression. Here is an example in the key of C Major:
Original Progression (II – V – I):
In the Key of C major our II-V-I progression would include D minor 7, G dominant 7 and C Major 7.
New Progression ( II – bII – I ):
If we substitute the V chord (G Dominant 7) for a Db Dominant 7 we get a new progression.
The chords are D minor 7, Db dominant 7 and C Major 7:
As well as the V7 chord (G7 in the key of C major) the mixolydian mode works well over a bII7 chord (Flat II7). Tritone substitutions are useful for creating a descending feeling because the resulting II, bII and I chord all have their root notes a semitone apart.
The recordings below show a II, V, I followed by a II, bII, I progression. The third example has the II-V-I repeated twice followed by the II, bII, I also repeated twice. There is a melody created using the G Mixolydian Mode over the top. (Keep in mind that the bII chord is out of key and so is used mostly as a passing chord).
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