Arcane Spell Failure Chance
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.
Casting an Arcane Spell in Armor: A character who casts an arcane spell while wearing armor must usually make an arcane spell failure check. . If the spell lacks a somatic component, however, it can be cast with no chance of arcane spell failure.
Shields: If a character is wearing armor and using a shield, add the two numbers together to get a single arcane spell failure chance.