Shields are an ancient yet extremely useful adjunct to body armor. Primitive shields are usually of wood or stretched hide, while more modern riot shields tend toward clear plastic. All shields require one free hand to use effectively.
Shields grant a base AC to their user; 13 for primitive and riot shields. If the bearer’s AC is already equal or better, the shield simply grants a +1 AC bonus. Primitive and riot shields are ignored by all weapons that ignore primitive armor.
Aside from their AC benefit, a shield also renders the bearer immune to the first instance of melee Shock damage they take each round, assuming the shield is effective against the weapon being used.