3e SRD:Armor Qualities
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Depending on a character's class, the character may be proficient in the use of all, some, or no armors, including shields. To wear heavier armor effectively, select the Armor Proficiency feats.
The cost of the armor.
The protective value of the armor. Bonuses from armor and a shield stack. This bonus is an armor bonus, so it does not stack with other effects that increase an armor bonus, such as the mage armor spell or bracers of armor.
This number is the maximum Dexterity bonus to AC that this type of armor allows. Heavier armors limit mobility, reducing a character's ability to dodge blows.
Even if a Dexterity bonus drops to 0, a character is not considered to have lost the Dexterity bonus.
Shields do not affect the maximum Dexterity bonus.
Armor Check Penalty
Anything heavier than leather hurts the ability to use some skills.
The armor check penalty number is the armor check penalty applied to certain skill checks.
Both armor check penalties apply when wearing armor and shield.
Nonproficient with Armor Worn
If armor is worn with which a character is not proficient, the character suffers the armor's armor check penalty on attack rolls and on all skill rolls that involve moving, including Ride.
Sleeping in Armor
If a character sleeps in a suit of armor with an armor check penalty of -5 or worse, the character is automatically fatigued the next day. The character suffers a -2 penalty on Strength and Dexterity, and can't charge or run.
Arcane Spell Failure
Armor interferes with the gestures that are needed to make to cast an arcane spell.
Casting an Arcane Spell in Armor
When casting an arcane spell while wearing armor, a character must make an arcane spell failure roll. The number in the Arcane Spell Failure is the chance that the spell fails and is ruined. If the spell lacks a somatic (S) component it can be cast without making the arcane spell failure roll.
Both spell failure chances should be combined when wearing armor and shield.
Medium and heavy armor slows a character down. The number on Table: Armor is a character's top speed while wearing the armor.
Shields do not affect speed.
The weight of the armor. Armor fitted for Small characters weighs half as much.