Triads

A triad is a three-note chord built by stacking two thirds. The lowest note in root position is the root, the middle note is the third, and the top note is the fifth. Four triad qualities exist, distinguished by the sizes of the two stacked thirds. A major triad has a major third on the bottom and a minor third on top (e.g., C–E–G). A minor triad has a minor third on the bottom and a major third on top (e.g., A–C–E).

A diminished triad stacks two minor thirds, producing a diminished fifth between root and fifth (e.g., B–D–F). An augmented triad stacks two major thirds, producing an augmented fifth (e.g., C–E–G♯). Major and minor triads are the most stable; diminished triads are tense and unstable; augmented triads are ambiguous and rarely function as stable chords in tonal music.

In any major key, triads built on each scale degree have predictable qualities: I, IV, and V are major; ii, iii, and vi are minor; vii° is diminished. In natural minor: i, iv, and v are minor; III, VI, and VII are major; ii° is diminished. These quality patterns are a direct consequence of the scale and are essential knowledge for harmonic analysis.