The basic theory (yuck - don’t like that word) behind the form of a minor chord is, for example, let’s take E major. E major contains the notes E (root), Ab (3rd) and B (5th). To form a minor chord you flat the 3rd, so in this case you flat the Ab. Now it becomes G (open) simply by lifting your finger off of the fret board. That is if you’re in the first (open) position.
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