AP® Music Theory · Unit 3 · Topic 3.1
Triad Qualities: Major, Minor, Diminished, Augmented
A triad's quality comes from the size of the two thirds stacked on top of each other: major, minor, diminished, or augmented, depending on whether each third is major (4 half steps) or minor (3 half steps).
A triad is two thirds stacked on a root: root, third, fifth. Each of those two thirds is either a major third (4 half steps) or a minor third (3 half steps), and the combination decides everything. Major third on the bottom plus minor third on top gives a major triad. Minor third on the bottom plus major third on top gives a minor triad. Two minor thirds stacked gives a diminished triad. Two major thirds stacked gives an augmented triad. That is the whole system: four qualities from two building blocks, arranged two ways plus two matching pairs.
Moving just one note by a half step changes which blocks you stacked, so it flips the sound. Take C E G, a major triad. Lower the E to E flat and the bottom third shrinks from major to minor: now you have a minor triad, C E flat G, same outer frame but a darker core. That one half step is the entire difference between bright and dark because it reclassifies the lower third.
This matters because every triad you build in a key starts from this same stacking logic, then gets labeled with a Roman numeral and read for inversion by looking at the bass note. Quality is the first fact you find about any triad; inversion and Roman numeral analysis both depend on knowing it correctly first.
Key terms
- Major third
- An interval spanning 4 half steps, used as a building block for triads.
- Minor third
- An interval spanning 3 half steps, used as a building block for triads.
- Triad
- A chord made of a root plus two stacked thirds, giving a root, third, and fifth.
- Triad quality
- The label (major, minor, diminished, or augmented) set by which sizes of thirds are stacked.
- Perfect fifth
- The 7 half step interval from root to fifth found in major and minor triads.
Watch it explained
- How it worksHow do the two stacked thirds decide whether a triad is major, minor, diminished, or augmented?
- Worked exampleFind the quality of the triads D F A, E G sharp B, B D F, and C E G sharp.
- Worked exampleWalk me through spelling an F sharp diminished triad and then an A flat augmented triad.
- Common mistakeWhy isn't a diminished triad just a minor triad with any lowered fifth you like?
Check yourself
End of topic.