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.

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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.