SRD:Armor Class (Creature Statistic)
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The Armor Class line gives the creature’s AC for normal combat and includes a parenthetical mention of the modifiers contributing to it (usually size, Dexterity, and natural armor). The creature’s touch and flat-footed ACs follow the combat-ready AC.
A creature’s armor proficiencies (if it has any) depend on its type, but in general a creature is automatically proficient with any kind of armor it is described as wearing (light, medium, or heavy), and with all lighter kinds of armor.