chord substitution (under construction)
Chord substitution is the technique of replacing a chord by another chord with a similar harmonic function. Chords from the major and minor keys can be interchanged; a commonly used substitute is a minor subdominant on the 4th degree used within a major key.
The 'tritone substitution' is common in both jazz and classical music. The tritone substitution can be performed by exchanging a dominant seventh chord for another dominant seventh chord which is a tritone away from it. In 18th- and 19th-century scale harmonizations, the augmented sixth chord on the 6xt degree used in the minor key downwoard motion, resembles very closely to a dominant substitute chord. The original chord symbol is B7b5, but has similar sounding pitches when written as F7b5, which the dominant chord a tritone away.
In Béla Bartók’s 'axis system' many options are possible to create substitute chords. Each harmonic function has four possible chords, which are visible by drawing a cross in the circle of fifths: