Seventh Chords and Their Qualities

A triad is stable: root, third, fifth, nothing straining against anything else. Stack one more third on top and you add a dissonant interval against the root, a seventh, that wants to resolve down by step. That restlessness is the whole personality of a seventh chord. It is not just a richer triad, it is a triad plus a built-in push toward the next chord.

Quality comes from two layers: what triad sits on the bottom, and what kind of seventh sits on top. A major triad with a minor seventh above the root gives a dominant seventh. A major triad with a major seventh gives a major seventh chord. A minor triad with a minor seventh gives a minor seventh chord. A diminished triad with a minor seventh gives a half-diminished seventh. A diminished triad with a diminished seventh gives a fully diminished seventh. Spelling one means building the triad first, then measuring the seventh above the root by half steps, not by eye.

This sits right after triad qualities and inversions because seventh chords use the same Roman numeral and bass-position logic, just with a fourth inversion possible (third factor in the bass) and a sharper sense of direction. The dominant seventh in particular becomes the engine of harmonic motion once cadences and part-writing start, since its tritone between third and seventh demands resolution to the tonic.