15.2The diatonic chords in C major

C major gives:

C Dm Em F G Am B°

or:

I ii iii IV V vi vii°

Now build chords from C natural minor:

Cm D° E♭ Fm Gm A♭ B♭

or roughly:

i ii° ♭III iv v ♭VI ♭VII

These minor-key chords can be “borrowed” while the overall piece remains in C major.