Shield bonus

Shield Bonus
A shield bonus is a specific class of bonus to Armor Class normally provided by shields. Two shield bonuses do not stack.

Shield bash causes the player to reliquish this bonus thus reducing their Armor Class unless they have the Improved shield bash feat.

Sources of shield bonus:
 * Feat: Two-weapon defense +1 (only when dual wielding)
 * Item: Light shield +1 (magical enhancements increase the shield bonus)
 * Item: Heavy shield +2 (magical enhancements increase the shield bonus)
 * Item: Tower shield +4 (magical enhancements increase the shield bonus; note that standard tower shields also impose a maximum dexterity bonus to AC of +2)
 * Spell: Shield +4