This a short guide about how I think about harmony. It’s a way of simplifying all the chords derived from a major scale into just 3 types/functions: Major I, Minor ii and Dominant V.
This has helped me navigate complex chord progressions and also is a great tool for discovering different chord options while composing.
Major Scale Chords
Here’s how it works.
Stacking the major scale notes in 3rds gives us 7 chord functions.
Simplifying to I, ii, and V
We can turn 7 into just 3 by combining the ones that share the same notes.
I, iii, and vi = Major I
The iii and vi are Em7 and Am7, respectively. Both of these chords function like versions of C, the Major I chord since they contain the same notes.
ii and IV = Minor ii
The IV chord, FMaj7, shares the same notes as Dm9, the ii chord.
The vii˚ chord Bº can be treated as a V7 chord since they share the same notes.
Because of these relationships, you can treat related chords in the same way. This is extremely for helpful quickly analyzing music built around major scale chords.
You can also apply this to minor scales and borrowed chords. As the complexity level of music increases, you simply are applying this concept to multiple keys instead of just one. It still all boils down to I ii and V, just in several keys.