Chords in a Key

What you’ll learn

Build a triad on every note of the C major scale, using only the scale’s own notes, and you get seven chords, one for each step. They’re numbered with Roman numerals, I to VII, counting up from the home note:

Step	1	2	3	4	5	6	7
Chord in C	C	Dm	Em	F	G	Am	B°
Numeral	I	ii	iii	IV	V	vi	vii°

Three are major: I, IV and V. A capital numeral means major. Three are minor: ii, iii and vi, written small. The last, vii° on B, stacks two minor 3rds: it’s called diminished, and the little circle marks it.

These seven are the key’s diatonic chords. I, IV and V, sometimes called the primary chords or the 1-4-5, are the strongest, and between them they fit most simple tunes.

The numbers stay the same in every key. In G major, I, IV and V are G, C and D; in F major they’re F, B flat and C. The key signature supplies the sharp or flat.

How the lesson works

There are three tasks:

1. Number the chord. A chord in C major: is it I, ii, iii, IV, V, vi or vii°? Ten right.
2. I, IV or V? A chord with a key signature, G or F major (and sometimes C). Find the key, count up to the chord’s root, and give its numeral. Ten right.
3. Chords for a tune. Choose I, IV or V for each part of Twinkle, Twinkle. Each button shows its chord’s three notes, so you can match them to the tune. Where two chords fit, either counts, but the first and last are always I, the home chord. When you’re done, the tune plays with your chords.