Why do some chords sound like they belong together? Because they come from the same scale. Let's build them, one on every note of the scale.
Take the C major scale. Start on C and take every other note: C, E and G. That's the C major . Now start on D and do the same: D, F and A, the D minor triad. Doing this on every note gives seven chords, all made only of notes from the scale:
The use the marks from Triads: Dm is D minor, B° is B diminished.
Musicians number these chords by the scale note they start on, using Roman numerals: I, II, III, IV, V, VI and VII for 1 to 7. A capital numeral means a major chord, a lowercase one a minor chord, and a small circle (°) a diminished chord. In every major the pattern is the same:
I ii iii IV V vi vii°
Three major chords (I, IV and V), three minor chords (ii, iii and vi) and one diminished chord (vii°). The I chord is ; the others lead away from it and back.
Pick any major key: the notes change, but the pattern stays the same. (Switch to minor to preview the next section.)
The chords of G major:
In a minor key the same idea gives a different pattern: i ii° III iv v VI VII. And since A minor uses the same notes as C major, its chords are the same seven chords, just numbered from A:
(This is the . Songs in minor often swap the v for a major V, such as E instead of Em in A minor, because it pulls home more strongly.)
On the circle of fifths, a key's three major chords sit right next to each other: C in the middle, F (IV) on one side and G (V) on the other. Their , Am, Dm and Em, are just inside.
Next: Chord Progressions