Chords in C Aeolian

C Aeolian is built from the notes C, D, Eb, F, G, Ab and Bb. Its scale carries 3 flats. Harmonising that scale in thirds gives seven chords — 3 majors, 3 minors and 1 diminished — starting on Cm as the i. The chord on the fifth degree is Gm rather than a major V, so C Aeolian has no leading tone and resolves by step and colour instead of by dominant pull — the sound that separates it from a plain C major.

Beyond those seven, C Aeolian commonly borrows C (I), Dm (ii), Em (iii), F (IV) and G (V) from the parallel major — the mixture chords that give a progression somewhere unexpected to go.