AP Music Theory Unit 3 Essentials

Every triad is a root with a third and a fifth above it — two thirds stacked. Whether each third is major or minor determines the quality: major, minor, diminished, or augmented. Add one more third on top and you get a seventh chord. Because the whole system is thirds, you can analyze any chord by re-stacking its notes into a neat pile and finding the bottom note (the root).

A Roman numeral names the scale degree of a chord's root, and its form shows the quality: uppercase for major, lowercase for minor, a ° for diminished, a + for augmented. This lets you describe a whole progression compactly (I–IV–V–I) and compare music across keys. The diatonic triads follow a fixed pattern in every major key (I ii iii IV V vi vii°) and minor key.

The bass note doesn't have to be the root. Figured-bass numbers count the intervals above the bass to show which chord tone is on the bottom: a triad is 6 in first inversion and 6/4 in second; a seventh chord is 6/5, 4/3, or 4/2 in its inversions. Reading these figures tells you both the chord and its position without a Roman numeral.

Adding a seventh to a triad creates five common qualities: major (MM7), dominant (Mm7), minor (mm7), half-diminished (ø7), and fully-diminished (°7). The dominant seventh is the most important — the tritone between its third and seventh creates tension that resolves powerfully to the tonic, setting up the harmony of Units 4–7.