Seventh chords
Add the 7th and the chord gets somewhere to go.
The chord that moves
Add a flat seventh to a major triad and the chord suddenly wants to go somewhere. The 7th is the note that makes harmony directional.
How we score it
dom7 1 3 5 ♭7 — the blues chord, pulls to the tonic maj7 1 3 5 7 — airy, jazzy, a little unresolved min7 1 ♭3 5 ♭7 — the mellow workhorse of soul
When to use it
Any time a plain triad feels too settled. The dominant 7th is the engine of the 12-bar blues.