Armor is what a creature can wear to gain Armor Class (AC), a number used to hit with in combat. AC Modifers can come from many sources to increase a player or creature's AC. These can be natural armor sources (like Chain Mail), or some magical (Ring of Protection).

The base AC of any creature, player, or item is: 10; is the starting point for a objects AC.

Anything effecting the Base AC of a object can only be effected once, and its effect(s) do NOT stack. This is also known as the AC Modifer, and is considered non-magical. One exception is wearing a shield, grants a base +2 AC.

Some special abilities give base AC. Examples: A Monk's unarmored defense; or mage armor. Most of these base modifers also have conditional elements attached to them to be active.

Magical AC modifers DO stack, and thus one reason why they are so valuable and sought after.