This document discusses chord quality substitution, where the quality (major, minor, dominant 7th etc.) of a chord can be changed while keeping the root note. It provides examples of substituting the ii and vi chords in a I-vi-ii-V progression to dominant 7th chords. It also discusses tritone substitution, where a dominant 7th chord can be substituted for the chord a tritone above it. Multiple chord quality substitutions can be combined, as demonstrated in a chord lick applying substitutions to a I-vi-ii-V progression in Bb major.