You already know a key gives you seven chords. Now you need a way to name them that doesn't care which key you're in — so a progression you learn once works everywhere. That naming system is Roman numerals.
Name a chord by its position
Instead of calling a chord by its letter (C, G, Am), call it by which scale degree it's built on. The 1st-degree chord is I, the 5th-degree chord is V, the 6th is vi, and so on. In scales and keys you saw how the whole family falls out of the scale; here we only care about the labels.
The clever part is upper- vs. lowercase, which encodes the chord's quality at the same time:
| Notation | Quality | Example (C major) |
|---|---|---|
| Uppercase — I, IV, V | major | C, F, G |
| lowercase — ii, iii, vi | minor | Dm, Em, Am |
| lowercase + ° — vii° | diminished | Bdim |
So the seven chords of any major key are always I ii iii IV V vi vii°. One line of symbols tells you both the position and the quality of every chord.
Why it's portable
Here's the payoff. Take the progression I – V – vi – IV — the backbone of countless pop songs. In C major it's C, G, Am, F:
Move to G major and the letters all change — but the Roman numerals don't:
C–G–Am–F and G–D–Em–C look nothing alike as letters, yet they're the same progression. Roman numerals strip away the key and leave only the shape — so you learn the pattern once and play it in all twelve keys just by counting off the new key's family.
The colours aren't part of the numeral. The indigo / rose / emerald shading above marks each chord's harmonic function — tonic (home), dominant (tension), subdominant (departure). That's a separate idea layered on top; the Roman numeral itself only tells you position and quality.
Minor keys start lowercase
Because the numeral reflects the tonic chord's quality, a minor key leads with a lowercase i: its family is i ii° III iv v VI VII (A minor: Am, Bdim, C, Dm, Em, F, G). Same idea, different starting quality — this series stays in major first, and minor slots in naturally once the frame is set.
In one line: a Roman numeral names a chord by its scale-degree position, with UPPERCASE for major, lowercase for minor, and ° for diminished — so I – V – vi – IV is the same progression in every key, only the letters changing. Next, see why some numerals pull toward others in harmony and why 1–4–5.