A stacks two thirds. Stack one more third on top and you get a four-note chord: a seventh chord, because the new note is the 7th above the root. Seventh chords sound richer, and they are everywhere in jazz, blues, soul and pop.
Which kind you get depends on the triad and on the size of the 7th. A major 7th is 11 half steps above the root, a minor 7th 10, and a diminished 7th 9.
| Chord | Name | Formula | Notes | Play |
|---|---|---|---|---|
| Cmaj7 | major seventh | 1 3 5 7 | C E G B | |
| C7 | dominant seventh | 1 3 5 ♭7 | C E G B♭ | |
| Cm7 | minor seventh | 1 ♭3 5 ♭7 | C E♭ G B♭ | |
| Cm7♭5 | half-diminished seventh | 1 ♭3 ♭5 ♭7 | C E♭ G♭ B♭ | |
| C°7 | diminished seventh | 1 ♭3 ♭5 𝄫7 | C E♭ G♭ B𝄫 |
The dominant seventh (C7) is a major triad with a minor 7th; it's the one simply called "seven". Its name comes from the 5th of a key, the dominant, because that's where it occurs naturally in a major key (so C7 is at home in F major, not C major). The diminished seventh's 7th is written B𝄫, not A: it's the 7th letter from C, lowered twice.
Stacking every other note of the C major scale, but four notes this time, gives the key's seventh chords. Again the pattern is the same in every major key:
Upper-case with maj7 are major sevenths, lower-case ones with 7 are minor sevenths. Only one chord in the key is a dominant seventh: the one on the 5th note, V7. The ø on vii means half-diminished (m7♭5): a diminished triad with an ordinary minor 7th on top instead of a diminished one.
G7 contains B and F. Those two notes are a apart (six half steps), the most restless interval there is. Both want to resolve, that is, move to a calm, stable note: B up a half step to C, F down a half step to E. C and E are in the C major chord, so G7 almost forces the music back to C.
Listen. Here the C chord is played as G C E, so the notes move as little as possible: B rises to C, F falls to E, and G stays where it is. (Changing which note is at the bottom is called an inversion: that's the next lesson.)
Many seventh chords have easy open . have seventh versions too. In the E shape, move the root on the D string down two frets for a 7 or m7 chord, or one fret for maj7. In the A shape, do the same with the root on the G string.