III Chords · Chapter 2
The seven triads of every key
Where I, ii, iii, IV, V, vi, vii° come from — and why their case follows a fixed pattern.
In Part II we learned that the seven Roman numerals of a major key follow a fixed pattern:
I, IV, and V are uppercase (major). ii, iii, vi are lowercase (minor). vii° is lowercase with a diminished mark.
We took that pattern on faith. This chapter proves it. The qualities aren’t arbitrary — they fall out of the major scale’s structure, exactly the way the sharps in G Sol major fell out of the W-W-H-W-W-W-H formula.
Everything you’ve learned so far converges in this chapter.
Building all seven chords in C major
Apply the “stack thirds” rule to every degree of C Do major. That gives us seven chords:
| Degree | Root | Chord notes | Quality | Name |
|---|---|---|---|---|
| I | C Do | C – E – G | major | C |
| ii | D Re | D – F – A | minor | Dm |
| iii | E Mi | E – G – B | minor | Em |
| IV | F Fa | F – A – C | major | F |
| V | G Sol | G – B – D | major | G |
| vi | A La | A – C – E | minor | Am |
| vii° | B Si | B – D – F | diminished | B° |
These are the seven triads of C major. They are the only chords that exist in the key of C major without any borrowed notes.
Why these specific qualities?
Look at the chord built on each degree and count the half steps from the root to the third, then from the third to the fifth:
- I (C major): C→E is 4 half steps (major 3rd). E→G is 3 (minor 3rd). Major third on the bottom → major chord.
- ii (D minor): D→F is 3 half steps (minor 3rd). F→A is 4 (major 3rd). Minor third on the bottom → minor chord.
- iii (E minor): E→G is 3, G→B is 4. Minor.
- IV (F major): F→A is 4, A→C is 3. Major.
- V (G major): G→B is 4, B→D is 3. Major.
- vi (A minor): A→C is 3, C→E is 4. Minor.
- vii° (B diminished): B→D is 3, D→F is 3. Two minor thirds → diminished.
This is where the diminished chord comes from: stacking two minor thirds produces a chord that contains a tritone between its root and fifth — the most dissonant interval in the system. We met the tritone in chapter 2.1 and called it “the devil in music.” Here it is, baked into the seventh degree of every major key.
In any other key
The same pattern in G Sol major:
| Degree | Chord notes | Quality | Name |
|---|---|---|---|
| I | G – B – D | major | G |
| ii | A – C – E | minor | Am |
| iii | B – D – F♯ | minor | Bm |
| IV | C – E – G | major | C |
| V | D – F♯ – A | major | D |
| vi | E – G – B | minor | Em |
| vii° | F♯ – A – C | diminished | F♯° |
The notes are different. The Roman numerals are identical. The qualities are identical. That’s what makes Roman numeral notation so powerful — the pattern is invariant across all keys.
Triads of a minor key
If you build triads on every degree of the natural minor scale instead, the pattern shifts:
In A La natural minor: A La m, B°, C Do , D Re m, E Mi m, F Fa , G Sol . Same construction rule, same underlying notes as C Do major (since A natural minor and C major are relatives) — but the chords appear in a different order because the tonal center has moved.
In harmonic minor, the raised 7th changes some of these — the V chord becomes major (and gains the leading-tone pull that the chapter on minor scales talked about). This is why most rebetiko progressions live in the tension between the natural minor chords and the harmonic-minor V.
We’ll see this in action in chapter 5.
Recap
- Stacking thirds on every degree of a scale produces seven chords.
- The qualities of these chords are fixed by the scale’s interval structure — they aren’t a memorization fact.
- For the major scale, the pattern is always maj, min, min, maj, maj, min, dim (I ii iii IV V vi vii°).
- The diminished chord on the 7th degree contains a tritone — the most dissonant interval in the system.
- The natural minor scale produces a different pattern (i ii° III iv v VI VII), and harmonic minor’s raised 7th creates a major V chord, the signature of Greek minor-key harmony.