Diatonic chords
The seven chords built from a key's own notes, in a quality pattern that never changes.
Take a key. Build a triad on each of its seven notes, using only notes from that key. The result is the set of chords that sound native to it.
In C major
| Degree | Root | Notes | Quality |
|---|---|---|---|
| 1 | C | C E G | Major |
| 2 | D | D F A | Minor |
| 3 | E | E G B | Minor |
| 4 | F | F A C | Major |
| 5 | G | G B D | Major |
| 6 | A | A C E | Minor |
| 7 | B | B D F | Diminished |
Nothing here was chosen. Every chord is forced by the rule "only notes from the key". The D triad is minor because D–F is a minor third, and F is in the key while F♯ is not.
The pattern is the same in every major key
major – minor – minor – major – major – minor – diminished
That is the single most useful pattern in tonal harmony. It holds in every major key, which is why a progression can move between keys and remain the same progression. See Roman numerals for the notation that makes this portable.
In G major the chords are G, Am, Bm, C, D, Em, F♯dim. Different letters, identical pattern.
In minor keys
Natural minor gives a different pattern:
minor – diminished – major – minor – minor – major – major
In A minor: Am, Bdim, C, Dm, Em, F, G.
There is a complication. That fifth chord is minor (Em), and a minor chord on the dominant does not pull towards the tonic — it has no leading tone. In practice, composers raise the seventh degree, turning Em into E major and giving the key a proper dominant.
This is precisely why the harmonic minor scale exists. It is a harmonic adjustment, and its name says so.
So minor keys in practice use a hybrid: mostly natural minor, with a raised seventh whenever the dominant chord appears.
Seventh chords work the same way
Stack one more third and the pattern holds. In a major key:
Imaj7 – iim7 – iiim7 – IVmaj7 – V7 – vim7 – viim7♭5
Note that the dominant seventh appears on exactly one degree, the fifth. That uniqueness is what makes it such a strong signal of which key you are in.
Non-diatonic chords
Chords using notes outside the key are chromatic. They are not mistakes — borrowed chords, secondary dominants and modulations all depend on them. But they are heard as departures, and that only works because the diatonic set establishes what "home" sounds like first.
Every key's full chord set is tabulated in the key reference.