The danger in learning chord extensions, chord substitutions and improvising is that we can lose the joy of what we are trying to do because we have made it too complicated. If you are young and have the time to put in then by all means learn it all.

When I studied chord theory I was able to use a different chord on every beat; wow, fantastic – but it sounded terrible. Just add one chord change and listen, no more.

Roman numerals are used because when we Say Chord “I” we are referring to the first step in all keys.

Key of C Major: I = C IIm7 = Dm7 V7 – G7 This is the most used progression in music. Learn it in all keys. If I was talking about Rock and Blues it would be: I = C IV F instead of IIm7 and V7 = G7. Let’s stay with: IIm7 V7 I = Dm7 G7 C.

One change: substitute a bII7 chord for a V7 chord Db7 instead of G7.

Possibilities in the key of C major.
Dm7 Db7 C
Dm7 G7 Db7 C

Whenever you see a V7 going to a I chord = G7 C make the change and listen to how it sounds. How it sounds is the final test.

John C.