Abilities or skills that "stack" add their effects together. For example, modifiers of +1 Save all and +2 Save against fear "stack" to create a total +3 to Save versus fear (in addition to the +1 to Save against everything else).
In most cases however, only the strongest effect applies. For example, having non-stacking +3 and +5 modifiers is only worth a +5 bonus, not +8.
Stacking follows a few predictable general rules:
- Bonuses usually (but not always) have a keyword, such as "enhancement" or "resistance", identifying what type of bonus it is. Bonuses of the same type do not stack.
- Dodge bonuses to AC are a partial exception to this. Dodge bonuses of different values do stack.
- Enhancement bonus to AC from armor stacks with Enhancement bonus to AC from shield.
- An "armor bonus" to AC granted by a non-armor and non-docent item does not stack with an actual armor. A "shield bonus" to AC does not stack with an actual shield.
- Some keywords like "exceptional bonus" stack with everything except themselves; a +1 exceptional bonus to Charisma will stack with everything except another +1 exceptional bonus to Charisma. It will stack with a +2 exceptional bonus to Charisma.
- Untyped bonuses stack. Note that a bonus without a keyword isn't necessarily an untyped bonus, the description might just be lacking.
- Penalties stack.
- Feat bonuses stack. A character with Lightning Reflexes (+2 bonus to Reflex saves) and Luck of Heroes (+1 bonus to all saves) will have a +3 Reflex save.
- Bonuses from character Enhancements are almost always untyped, and thus stack. They are not automatically "enhancement bonuses".
- Benefits from the same family, like Fortification or False Life, do not stack.
- Damage Reduction does not stack (eg. Stoneskin), except for the added DR that comes from active blocking (therefore it's not a waste to use it together with Barkskin which grants an increase to AC, i.e. an unrelated modifier).
- If you use a bow or crossbow, bonuses and properties on your weapon do not stack with ones from the same category on your ammunition. For example, using +3 bolts of lesser undead bane in a +1 flaming crossbow of lesser undead bane gives your shots +3 to attack and damage (not +4), flaming damage, and lesser undead bane effect once.
Players usually try to maximize the potential of their characters in a given area by giving them a collection of bonus with different keywords. Achieving a high Armor Class is a good example of this strategy.
 Spell damage
For information about stacking of various spell damage effects, refer to Spell Damage.