With minor keys, the nature of the parent scale has an effect on the quality of diatonic chords, but not their function. For instance, tonic chords in a major key are I, iii and vi, but in a minor key are i, III and VI. Dominant chords in a major key are V and viiº, but in a minor key are v and VII if the natural minor is used as the parent scale, or V and viiº if the harmonic minor is used, due to the presence of its sharpened 7th degree.
TALKIN ‘BOUT A SUBSTITUTION
Any chord that can take the place of another and perform the same harmonic function is known as a ‘subsidiary’ chord. Subsidiary chords can be used to liven up mundane chord progressions that feature mainly primary chords (I, IV and V), either by complete substitution of the original or by changing halfway through a long primary chord, speeding up the rate at which the chords change (harmonic rhythm).