Diatonic chords and Roman numerals
Why is ii always minor and V always major? Step through a scale, stacking every other note, and watch the pattern appear.
I C
Degree 1: C – E – G — major
Thirds from the bottom: major 3rd (4)minor 3rd (3)
What the explainer shows
A triad is three notes, each a third apart. On a scale, a third means “skip one note”: from C, skip D, land on E; skip F, land on G. Because the scale has half steps in some places and whole steps in others, some of those thirds are major (4 half steps) and some are minor (3).
- Major 3rd + minor 3rd = a major chord (I, IV, V in a major key).
- Minor 3rd + major 3rd = a minor chord (ii, iii, vi).
- Minor 3rd + minor 3rd = a diminished chord (vii°).
Change the key and the letters move, but the qualities don’t — the pattern of whole and half steps is the same in every major key. That is exactly why Roman numerals work: “I–V–vi–IV” describes the same progression whether you play it in C (C–G–Am–F) or E♭ (E♭–B♭–Cm–A♭).
Harmonic minor’s raised 7th
Switch to harmonic minor and watch degree 5. Raising the 7th note turns the minor v into a major V, and vii becomes a diminished chord a half step below the tonic. That leading tone is what makes minor-key cadences sound final.
Put the numerals to work in the chord progression generator, where every chord is tagged with its numeral and function.
Questions
What are diatonic chords?
Chords built only from the notes of one scale. Stack every other scale note on each degree and you get seven triads. In any major key they are always major, minor, minor, major, major, minor, diminished — I ii iii IV V vi vii°.
What is Roman numeral analysis?
Labelling chords by the scale degree of their root instead of by letter name. Upper-case numerals are major chords, lower-case are minor, ° is diminished and + is augmented. It lets you describe a progression — say I–vi–IV–V — independently of key, so you can transpose it anywhere.
What are the diatonic chords in a minor key?
In natural minor: i ii° ♭III iv v ♭VI ♭VII. Composers usually raise the 7th (harmonic minor) to make the v chord major (V), because a major V with its leading tone pulls much harder back to i.
Why is the vii chord diminished?
Its root and fifth are only six half steps apart. In C major the chord on B is B–D–F, and B to F is a tritone rather than a perfect fifth — the only such pair in the major scale.