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 D20
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This article is a D20 reference page
The System Reference Document is a comprehensive toolbox consisting of rules, races, classes, feats, skills, various systems, spells, magic items, and monsters compatible with the d20 System version of Dungeons & Dragons and various other roleplaying games from Wizards of the Coast.
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Armor Class
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.
System Reference Document -> Reading Creature Entries