Chord Substitutions
Substitutions can be functional as in Tritone Substitutions or minor 3rd substitutions. Or they can be non-functional or contextual substitutions (as David Baker calls them) as in Coltrane Changes or superimposing cycle changes over the blues or rhythm changes.
Functional means that a dominant chord is substituting for a dominant chord and is still functioning in the same way the original chords did.
Non-functional or contextual substitution involves superimposing chords that have their own logic that is independent of the original chords and doesn't necessarily relate to the original chords. In both the cycle and Coltrane changes, a sequence of chords is superimposed over the original changes in a way that the chords arrive at the desired destination, but they take a path which is mostly unrelated to the original path. The sequential movement becomes its own reason for existence.
Another example of contextual substitution is given with the blues progression variations:
|| F7 | Eb7 | Db7 | B7 | Bb
I IV
Beginning on I, the target is IV, moving down in whole steps sets up a half step resolution.
Functional means that a dominant chord is substituting for a dominant chord and is still functioning in the same way the original chords did.
Non-functional or contextual substitution involves superimposing chords that have their own logic that is independent of the original chords and doesn't necessarily relate to the original chords. In both the cycle and Coltrane changes, a sequence of chords is superimposed over the original changes in a way that the chords arrive at the desired destination, but they take a path which is mostly unrelated to the original path. The sequential movement becomes its own reason for existence.
Another example of contextual substitution is given with the blues progression variations:
|| F7 | Eb7 | Db7 | B7 | Bb
I IV
Beginning on I, the target is IV, moving down in whole steps sets up a half step resolution.