Most chord subs are based on the following possibilities:
1. For most dominant 7th chords, you can sub or add the minor7th chod a perfect 4th below and vice versa. NOtice that their respective scales have exactly the same notes:
F7 = Cm7 F7
- C dorian, F mixolydian
2. For most dominant 7th chords, you can sub the dom7 chord a tritone away. This is commonly referred to as the tritone sub. Notice that the 3rds and 7ths are interchangeable.
F7 = B7
3. Passing chords maybe inserted between the normally occuring chords. This is usally done by means of a stepwise bass line:
FM7 F#dim7 — Gm7 Abdim7 — Am D7
or through the use of chord cycles, typically in a ii-V relatinship.
These chord cycles move chromatically by whole steps, 3rds, or 4ths as long as they resolve logically to the IV chord in the 5th bar and the ii or V chord in the 9th bar.
4. Melody permitting, you can change the tonality from a dominant key to a major key or vice versa.
5. You may drop the root of the chord, leaving the upper extensions or add a new root underneath the existing chord.
6. Melody permitting, you may substitute any of the dominant 7th chords whose roots belong tothe same fully diminished 7th chord
F7 = Ab7 = B7 = D7
This is due to the fact that all four chords share the same notes in their respective diminished half step scales.
7. In th emodern blues form, you may leave the key center or superimpose any tonaligy, as long as the integrity of the 3-phrase structure is maintained.