The chords Em, Am, and D are all in the key of Em, but what key is it/what scale do you use when you made the D a minor chord?
Em = E G B
Am = A C E
D = D F#A
=>notes = A B C D E F# G = 1 sharp = E minor

change D-> Dm
D= D F A

=>notes = A B C D E F G = 0 sharp/flat = A minor

EDIT: ##additional comments - feel free to ignore this if it seems confusing##
of course 3 chords don't necessarily define whether it's A minor or C major or anything else but for the purposes of this question we won't go there in any detail beyond saying "you would establish it being C major or A minor from the resolution".
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Last edited by doive at Apr 11, 2011,