Seventh chords

Stack one more third on top of a triad and you get a seventh chord — four notes: root, third, fifth and seventh.

1. F
   minor
2. D
   third
3. B
   major
4. Groot

The five you need

The naming trips people up: a "seventh chord" with no other qualifier means a dominant seventh — a major triad with a minor seventh. C7 is not C with a B; that is Cmaj7.

Why the dominant seventh pulls

C7 contains E and B♭ — six semitones apart, a tritone. That interval is unstable, and it resolves in a specific way: E wants to rise a semitone to F, B♭ wants to fall a semitone to A.

Both of those notes are in an F major chord. So C7 leans towards F, hard. That single mechanism is the engine of tonal harmony, and it is why the chord built on the fifth degree of a key is called the dominant.

The others in one line each

Major seventh — a major triad with the seventh a semitone below the octave. Soft, unresolved, everywhere in jazz and soul.

Minor seventh — a minor triad with a minor seventh. The default minor chord in jazz and pop; much less mournful than a plain minor triad.

Half-diminished — a diminished triad with a minor seventh. Written m7♭5 or with the ø symbol. Its usual job is the ii chord in a minor key.

Diminished seventh — three stacked minor thirds. It divides the octave evenly, so every inversion sounds like a root position and it can resolve almost anywhere. Useful as a pivot.

Sevenths in a key

Build sevenths on each degree of a major scale and, like triads, the qualities follow a fixed pattern:

Imaj7, ii m7, iii m7, IVmaj7, V7, vi m7, vii m7♭5

Notice that the dominant seventh occurs naturally on only one degree — the fifth. That uniqueness is what makes it such a strong signal of the key.