Stacking occurs when multiple modifiers are cumulative, possibly up to a cap. This is in contrast to only the best modifier being used (overlapping) and to earlier modifiers being forgotten (replacing). The rules that determine when things stack are somewhat haphazard, with many exceptions. Some of these take into consideration the source of an effect, while most are based on the type of effect.
Examples of things that do stack:
- Ability score bonuses stack up to +12 (bonus coming from items, spells, abilities).
- Armor class bonuses of different types stack (armor, shield, deflection, natural, dodge).
- Dodge bonuses to armor class stack up to +20.
- Attack bonuses stack, with some types capped at +20 (bonus coming from weapons attack bonus/enhancement, spells, abilities)
- Damage immunity/vulnerability stacks up to +100%/-100% (bonus coming from items, spells, abilities, races, feats).
- Saving throw bonuses stack up to +20 (bonus coming from items, spells, abilities).
- Skill bonuses stack up to +50 (bonus coming from items, spells, abilities).
Examples of things that do not stack:
- Armor class bonuses of the same type, other than dodge, do not stack.
- Concealment does not stack.
- Damage reduction does not stack.
- Damage resistance does not stack (except for resistance from feats).
- Spell resistance does not stack.
Penalties follow the same rules, but are treated separately. That is, for things that do not stack, the largest bonus and the largest penalty apply. When there is a cap, the cap usually applies after the bonus and penalty are combined (or bonuses and penalties, if the thing in question stacks). For example, if a character with two +12 dodge armor class bonuses (stacking to +24, but capped at +20) acquires a -2 dodge penalty, the result will be no change in armor class (since +12 + 12 – 2 = +22, which then gets capped to +20).