Tritone Substitution
Another common method of substitution is to use a chord whose root is a tritone (flattened 5th or augmented 4th) above that of the original chord. This is known as tritone substitution.
Like other methods of substitution, the reason this works is that there are notes common to both chords. Shown below are the chords G7 and Db7. As you can see, the note F occurs in both chords. The note B is enharmonically the same as Cb which is the 7th of Db7, so this note is in both chords too.
In the following example, Db7 is substituted for G7, creating a 2 b2 1 progression in the key of C instead of a standard 2 5 1. Notice how the notes Db and Ab in the Db7 chord give the melodic line a different flavor to a 2 5 1 line using a G7 chord.
Tritone substitutions are most commonly used with dominant chords, but can be used with any chord type and on any degree of a scale. In some cases, every chord except for 1 can be a tritone substitution. The following example shows a 1 6 2 5 progression and then a second version where b3 is substituted for 6 , b6 is substituted for 2, and b2 is substituted for 5. This type of substitution throughout the progression is sometimes called backcycling. Notice that the chord type has been changed from min7 to maj7 for the Eb and Ab chords. This is because the major 7ths contain notes which are common to the key of C.
