harmonizations II: chord substitutions
Chord substitution is the technique of replacing a chord with another chord that has a similar harmonic function. For example, chords from the major and minor keys can be interchanged.
The 'tritone substitution' in dominant chords is common in both jazz and classical music. A dominant seventh chord can replace another dominant seventh chord that is a tritone away from it. In 18th- and 19th-century scale harmonization, the augmented sixth chord on the 6th degree used in the minor key very closely resembles a substitute dominant chord. In the example below, the original chord symbol is B7b5, but has similar sounding pitches when written as F7b5, which is the dominant chord a tritone away.
For further reading: The Jazz Piano Book (1989) and The Jazz Theory Book (1995) by Mark Levine (1938-2022).
In Béla Bartók’s 'axis system', many options are available to create substitute chords. Each harmonic function has four possible chords substitutions, which are found by drawing a cross in the circle of fifths.