On this site, and in music generally, the chords that belong to a key are named with Roman numerals like I, IV, and V. If those symbols are unfamiliar, here's a quick primer.
What are Roman numerals?
Roman numerals are a way of writing numbers with letters, invented in ancient Rome and still in use for things like clock faces, book chapters, and the names of kings and queens. A few letters stand for fixed values, and we combine them to build up a number.
Three of those letters are all we need here:
- I is 1.
- V is 5.
- X is 10.
Reading the numerals
Numerals are read from left to right, adding their values as we go. There's one twist: a smaller numeral placed before a larger one is subtracted instead of added.
- Repeat a numeral to add: II is 2, III is 3.
- A smaller numeral after a larger one adds: VI is 6, VII is 7.
- A smaller numeral before a larger one subtracts: IV is 4 (a 1 before the 5), IX is 9 (a 1 before the 10).
For chords we never have to count past seven, one numeral for each note of the scale.
I, II, III, IV, V, VI, VII = 1, 2, 3, 4, 5, 6, 7
How chords use them
In a key, each of the seven chords is numbered by the scale note it's built on. Writing those numbers as Roman numerals keeps them clearly apart from the other numbers music is full of, like string, fret, and finger numbers. A roman numeral almost always means a chord.
The case of the numeral carries one more piece of information, the chord's quality:
- Uppercase is a major chord: I, IV, V.
- Lowercase is a minor chord: ii, iii, vi.
- Lowercase with a small circle is a diminished chord: vii°.
So the seven chords of a major key, written in order, are I, ii, iii, IV, V, vi, vii°. The numeral tells us where each chord sits in the key, and its case tells us the chord's quality at a glance.