The four basic 7th chords

Concept

Stack one more 3rd on a triad and you get a 7th chord. The four flavors: maj7 (1 3 5 7 — dreamy, e.g., Cmaj7), m7 (1 b3 5 b7 — smooth, e.g., Am7), dom7 (1 3 5 b7 — bluesy/tense, e.g., G7), and m7b5 (1 b3 b5 b7 — half-diminished, e.g., Bm7b5). Each has a distinct emotional character.