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.
The major rule considering the source of effects involves spells. As a rule, spells overlap – so do not stack – with themselves, but do stack with different spells, with few exceptions that are noted in the individual spell articles. That is, multiple castings of the same spell – such as cat's grace – will provide only the largest bonus of each individual casting. However, different spells – such as owl's wisdom and owl's insight – usually stack if they provide a type of effect that normally stacks. This leads to a peculiarity with some feats that mimic spells, as those feats are typically their own spells as far as the game engine is concerned. (For example, the bull's strength spell and the bull's strength feat are technically different spells, so they stack.)
When the effects in question come from different spells or from non-spells, the determination of whether or not they stack often depends on the type of effect.
Examples of things that do stack:
- Ability score bonuses stack up to +12.
- Armor class bonuses of different types stack.
- Attack bonuses stack, with some types capped at +20
- Damage immunity stacks (additively) up to 100%.
- Dodge bonuses to armor class stack up to +20.
- Saving throw bonuses stack up to +20.
- Skill bonuses stack up to +50.
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).