Diatonic Tetrad Harmony
Connect the 7th chords to the degrees of the major scale. Adding the 7th gives the triads the character of jazz and soul, and learning to find them in different shapes builds a toolkit that covers over 90% of modern music.
Why
The 7th adds color to every diatonic chord
Knowing which 7th chord belongs to each degree is a powerful skill
Each 7th chord contains and connects two of the key's triads
The dominant 7th is the only one with a tritone, and its pull toward the root creates a strong sense of resolution