Tritone Substitions & Substitute Chords

FLAT 5 SUBSTITUTES (tritone substitutes)

A dominant 7th chord is characterised by the tension set up in the tritone interval between the 3rd and the 7th, which has a tendency to resolve to the root and 3rd of the tonic (ex 8a).

ex 8a: Tritone resolution

A b5 substitute is derived by chromatically altering the fifth (ex 8b-1). If this chord is used in its second inversion the flattened fifth becomes the bass note (ex 8b-2), the interval between the bass note and the 3rd is an augmented 6th (hence the classical term augmented 6th chord). However in jazz the notes are enharmonically changed to create another V7 chord in root position (ex 8b-3).

ex 8b: Derivation of b5 substitute.

In jazz this has become known as the b5 (or tritone) substitute and in practical use the dominant chord whose root is a b5 away from a dominant can be used as a substitute, whether or not its 5th is flattened. The important consideration is not its classical derivation but the fact that it shares the all important tritone (C/F# in the case of D7/Ab7) with its substitute. Note that in this case it is allowable to change enharmonic spellings.

Because of the tritone the two most important notes of any dominant chord are the 3rd and 7th, (Apart from the root which obviously defines the root of the chord).

Note that the same tritone is present in every pair of V7 chords whose roots are a b5th apart. (ex 8c)

ex 8c: Matching tritones in b5 substitutes.

One should be aware that a b5 substitute may be unsatisfactory if the melody is on the 5th or 9th of a V7 chord due to the altered tendency of the newly created chord. For example the 5th of a dominant 7 chord may have a tendency to resolve down a whole tone to the tonic (ex 8d-1, but the natural tendency of a b9 (where the melody will be if a b5 substitute is created - ex 8d-2) is to resolve down or sometimes up by a semitone.

ex 8d: Incorrect use of tritone substitution.

In a cycle of fifths (ex 8e-1), if every alternate V7 is substituted by a b5 substitute the result is a chromatic cycle (ex 8e-2)

ex 8e: Chromatic root movement created by tritone substitution on alternate chords of a cycle of fifths.