AP® Music Theory · Unit 3 · Topic 3.2
Diatonic Triads in a Major Key
Stacking thirds on every scale degree of a major key always produces the same pattern of chord qualities: I, IV, V major; ii, iii, vi minor; vii diminished. The pattern comes straight from the scale's fixed arrangement of whole and half steps.
Build a triad on each scale degree by stacking the next two notes a third above, using only notes already in the key. Because a major scale has whole and half steps in one fixed order, every stack of thirds lands on a predictable quality. You never have to guess: the pattern is the same in every major key, note names aside.
The quality of each triad depends on the size of the two stacked thirds. A major third plus a minor third above it gives a major triad. A minor third plus a major third gives a minor triad. Two minor thirds stacked gives a diminished triad. Where the half steps fall in the scale (between scale degrees 3-4 and 7-8) decides which combination appears on each root. That half step between 7 and 8 is the key detail: it shrinks the fifth above the leading tone to a diminished fifth, so vii is the one diminished triad in the set.
This fixed quality pattern, I-ii-iii-IV-V-vi-vii-dim, is what Roman numeral analysis is built on: capital numerals for major, lowercase for minor, a degree sign for diminished. It also sets up inversions and seventh chords later in the unit, since the same stacked-thirds logic extends to a fourth note, and the diminished vii triad becomes the core of the leading tone seventh chord.
Key terms
- Diatonic triad
- A three-note chord built from stacking thirds using only notes already in the key.
- Triad quality
- Whether a triad is major, minor, diminished, or augmented, based on the sizes of its stacked thirds.
- Leading tone
- Scale degree 7, a half step below the tonic, which forms the diminished vii triad.
- Diminished fifth
- A fifth one half step smaller than perfect, formed between the leading tone and scale degree 4.
- Roman numeral analysis
- A system labeling each diatonic chord by scale degree and quality, using case and symbols to show major, minor, or diminished.
Watch it explained
- How it worksHow does the major step pattern force I, IV, and V to be major while ii, iii, and vi come out minor?
- Worked exampleFind and spell all seven diatonic triads in G major with their qualities.
- Common mistakeWhy isn't the chord built on the seventh degree major like the other bright ones?
- Ties togetherHow does the diminished triad on scale degree 7 connect to the tritone inside the scale?
Check yourself
End of topic.