About this issue
Why this chart.
Seventh chords add color and tension to a triad by including the seventh note of the scale. They're the harmonic vocabulary of jazz, blues, gospel, and any music that wants to sound sophisticated rather than primary-color.
Five flavors matter: dominant 7 (the V chord of every key, full of pull), major 7 (dreamy, sophisticated, the tonic of jazz ballads), minor 7 (the ii in ii-V-I), half-diminished (the ii of a minor key), and diminished 7 (passing tension).
Once you can play these five seventh-chord types in every key, you have access to the entire jazz standard repertoire. Autumn Leaves, Fly Me to the Moon, All the Things You Are — they're all built from these voicings.
How to use it
Replace V with V7 in any progression to add forward momentum. Replace I with Imaj7 to add gentleness. The ii chord of a major key is almost always better as ii7 (m7).