How to find the chords in a key
Stack a scale in thirds and every note becomes a chord. The seven chords of C major (C, Dm, Em, F, G, Am, Bdim), the pattern that works in every key, and the minor-key version.
The rule
Take the seven notes of the key's scale. Build a chord on each note by stacking every other scale note on top of it — the root, then the note two steps up, then two more. Those seven chords are the chords in the key, called the diatonic chords, and a song in that key is made mostly of them.
Worked example: C major
C major is C D E F G A B. Stack thirds on each note and you get:
- C — I, built on C
- Dm — ii, built on D
- Em — iii, built on E
- F — IV, built on F
- G — V, built on G
- Am — vi, built on A
- Bdim — vii°, built on B
Three are major (I, IV, V), three are minor (ii, iii, vi), one is diminished (vii°). That major-minor-minor-major-major-minor-diminished pattern is the same in every major key — only the letter names change. In G major it gives G, Am, Bm, C, D, Em, F#dim.
As seventh chords, C major yields Cmaj7, Dm7, Em7, Fmaj7, G7, Am7, Bm7♭5.
The numerals are the point
Players write those chords as Roman numerals — I ii iii IV V vi vii° — uppercase for major, lowercase for minor, because the numerals are true in every key. A I–V–vi–IV progression is C – G – Am – F in C and G – D – Em – C in G, and a player who knows the numerals can play it in either without thinking.
Minor keys
A minor key uses the natural minor scale, so the pattern shifts: A minor (A B C D E F G) gives Am (i), Bdim (ii°), C (III), Dm (iv), Em (v), F (VI), G (VII). Notice they are the same seven chords as C major, because A minor and C major share every note — the relative major and minor. What differs is which chord is home.