3e SRD:Arcane Spells and Armor
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Arcane Spells and Armor
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.