Seventh chords, and the tritone that moves harmony
A fourth stacked note turns the chord from a state into a direction, and a single internal interval explains why.
Stack one more third on the triad and the chord gains a seventh. The change looks small and is not: the triad describes a state, the seventh chord describes a direction.
The five that matter
Combining the quality of the triad with the quality of the seventh, five seventh chords cover practically the whole tonal repertoire:
Major seventh — . Major triad, major seventh. At rest, luminous, with no wish to go anywhere.
Minor seventh — . Minor triad, minor seventh. Stable too, but closed.
Dominant — . Major triad, minor seventh. Unstable, and the instability has an address.
Half-diminished — . Diminished triad, minor seventh. The chord of the minor second degree.
Diminished — . Everything diminished, the seventh included. Symmetrical, with no obvious root.
The tritone
What sets the dominant apart from the others fits in one interval.
In , G, B, D, F, measure from B to F: six semitones, a . That is the tritone, and it is the reason the chord cannot stay still.
The two notes resolve in opposite directions, by a semitone each:
- B rises to C — it is the leading tone
- F falls to E — it is the seventh
Two semitone resolutions in contrary motion. It is the strongest gesture in tonal harmony, and it sits entire inside one chord.
Only one degree has it
In the major diatonic field with sevenths, exactly one chord is dominant:
The fifth degree, and no other. Not by choice: it is the only place in the scale where a major triad meets a minor seventh.
The half-diminished and the diminished
Two that look alike on paper and do different jobs.
— B, D, F, A — is the seventh degree of major and the second degree of minor. It has one tritone, between B and F, and so it works as preparation.
— B, D, F, A♭ — has two tritones: B–F and D–A♭. Stacked only of minor thirds, it is symmetrical: any of its four notes can be taken as the root, and the chord changes its meaning without changing its sound.
Where the seventh changes the text
Compare the same functions with triads and with seventh chords:
The second version is not more sophisticated for ornament's sake. The sevenths create voice leading: the seventh of one chord becomes the third of the next, and the line walks by semitone instead of leaping. That is what makes the II–V–I sound inevitable.
What to take from this
The fourth note turns state into direction. The dominant chord is the only one in the major field that carries a tritone, and that tritone is the engine: two notes a semitone from their destination, pulling in opposite directions. All of functional harmony is built on that tension and its resolution.