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If you know I, ii, iii, IV, V, vi in C then you'll be able to play tunes in A natural minor. Add III to cover A harmonic minor, and II to cover A melodic minor

In real life though the key usually depends on the singer's vocal range, so you won't really get away with learning just one key

[tarentel]

In A harmonic minor the III is already a C, you can make it augmented but that is not very common. You generally change the v to a V and keep the VII diminished in both cases where as it is major in the natural minor.

[PhaseLockk]

He was saying that the III of C major is the V of A minor, and so if you are planning to play in A minor using a set of chords pulled from C major, you may want to add a III alongside the iii.

[tarentel]

Ah, when phrased that way it makes sense. I misunderstood. We're just saying the same thing in two different ways.

[laGrenouille]

Yes, that's a good point in the case of six keys (I think my point holds for just learning three), but it is not the one made that the article seemed to make.