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Natural AC is 9. Does armor adds a bonus or armor has it's own AC?
A PC with padded armor is AC 8, but how about an orc, who is natural AC 6? Does a padded armor worsen his AC from 6 to 8 (unlikeley), does he is still AC 6 (doesn't make sense), or does he get a bonus, thus being now AC 5?
And what if the same orc is wearing a Full Plate armor (AC 1)? Is he now AC -2?
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As a general (unofficial) rule of thumb, an unarmoured orc is AC 6, an orc in leather or studded armour might be AC 5, and an orc in (orcish) plate mail would be AC 4. Were an orc to be girded in manmade full plate armour, it would be AC 1.