D20:Armor Qualities Rules

To wear heavier armor effectively, a character can select the Armor Proficiency feats, but most classes are automatically proficient with the armors that work best for them.
Armor and shields can take damage from some types of attacks.
Here is the format for armor entries (given as column headings on Table: Armor and Shields, below).

The cost of the armor for Small or Medium humanoid creatures. See Armor for Unusual Creatures, below, for armor prices for other creatures.

Each armor grants an armor bonus to AC, while shields grant a shield bonus to AC. The armor bonus from a suit of armor doesn?t stack with other effects or items that grant an armor bonus. Similarly, the shield bonus from a shield doesn?t stack with other effects that grant a shield bonus.

This number is the maximum Dexterity bonus to AC that this type of armor allows. Heavier armors limit mobility, reducing the wearer?s ability to dodge blows. This restriction doesn?t affect any other Dexterity-related abilities.
Even if a character?s Dexterity bonus to AC drops to 0 because of armor, this situation does not count as losing a Dexterity bonus to AC.
Your character?s encumbrance (the amount of gear he or she carries) may also restrict the maximum Dexterity bonus that can be applied to his or her Armor Class.

Shields do not affect a character?s maximum Dexterity bonus.

Any armor heavier than leather hurts a character?s ability to use some skills. An armor check penalty number is the penalty that applies to Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Sleight of Hand, and Tumble checks by a character wearing a certain kind of armor. Double the normal armor check penalty is applied to Swim checks. A character?s encumbrance (the amount of gear carried, including armor) may also apply an armor check penalty.

If a character is wearing armor and using a shield, both armor check penalties apply.

A character who wears armor and/or uses a shield with which he or she is not proficient takes the armor?s (and/or shield?s) armor check penalty on attack rolls and on all Strength-based and Dexterity-based ability and skill checks. The penalty for nonproficiency with armor stacks with the penalty for nonproficiency with shields.

A character who sleeps in medium or heavy armor is automatically fatigued the next day. He or she takes a ?2 penalty on Strength and Dexterity and can?t charge or run. Sleeping in light armor does not cause fatigue.

Armor interferes with the gestures that a spellcaster must make to cast an arcane spell that has a somatic component. Arcane spellcasters face the possibility of arcane spell failure if they?re wearing armor. Bards can wear light armor without incurring any arcane spell failure chance for their bard spells.

A character who casts an arcane spell while wearing armor must usually make an arcane spell failure roll. The number in the Arcane Spell Failure Chance column on Table: Armor and Shields is the chance that the spell fails and is ruined. If the spell lacks a somatic component, however, it can be cast with no chance of arcane spell failure.

If a character is wearing armor and using a shield, add the two numbers together to get a single arcane spell failure chance.

Medium or heavy armor slows the wearer down. The number on Table: Armor and Shields is the character?s speed while wearing the armor. Humans, elves, half-elves, and half-orcs have an unencumbered speed of 30 feet.
They use the first column. Dwarves, gnomes, and halflings have an unencumbered speed of 20 feet. They use the second column. Remember, however, that a dwarf ?s land speed remains 20 feet even in medium or heavy armor or when carrying a medium or heavy load.

Shields do not affect a character?s speed.

This column gives the weight of the armor sized for a Medium wearer. Armor fitted for Small characters weighs half as much, and armor for Large characters weighs twice as much.

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System Reference Document -> Equipment