To stack is to combine for a cumulative effect. In most cases, modifiers to a given check or roll stack if they come from different sources and have different descriptors (or no descriptors at all), but do not stack if they have the same descriptors or come from the same source (such as the same spell cast twice in succession). If the modifiers to a particular roll do not stack, only the best bonus or worst penalty applies, i.e., they overlap.
Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified. Spell effects that do not stack may overlap, coexist independently, or render one another irrelevant, depending on their exact effects.
Turn Undead stacking: The cleric class and the paladin class both have turn undead. Clerics have an undead turning level equal to their cleric class level, and paladins have undead turning level equal to their paladin class level minus three. The prestige class blackguard also has turn undead, 2 levels below his class level. Undead turning between paladin and cleric levels stack. For example: a character with 10 cleric levels and 8 paladin levels gain 10 levels of undead turning from his cleric class levels and 5 levels of undead turning from his paladin levels, for a total of level 15 undead turning. Blackguard undead turning and paladin undead turning levels do not stack however, but uses the highest undead turning level between the two. Examples:
- A level 10 cleric / 8 paladin / 5 blackguard has a level 15 turn undead; 10 from his cleric levels and 5 from his paladin levels, ignoring the 3 turn undead levels from his blackguard class
- A level 10 cleric / 5 paladin / 5 blackguard has a level 13 turn undead; 10 from his cleric levels and 3 from his blackguard levels, ignoring the 2 levels from his paladin class
Armor stacking: The Armor Class (AC) in NWN2 is based on a lot of individual stacking and non-stacking Armor Class bonuses. The following bonuses stack with the other bonuses, but not with bonuses of the same type:
- Size modifier (+1 for small creatures, 0 for medium creatures, -1 for large creatures)
- Armor bonus (A body armor's armor bonus includes it's enchantment bonus)
- Shield bonus (A shield's shield bonus includes it's enchantment bonus The highest current shield bonus)
- Dexterity bonus (The current dexterity modifier)
- Deflection bonus
- Natural armor bonus (Racial/Class bonuses; the Pale Master's Tough as Bone feat or Red Dragon Disciple's natural armor feats)
- Natural armor enchantment bonus (Spell bonuses from spells like Barkskin)
Only dodge and generic bonuses stack with themselves;
- Dodge bonuses (From sources such as Boots of the Sun Soul, feats such as Dodge and Weapon Expertise as well as the tumble skill's AC bonus)
- Generic bonuses (From the feat Luck of Heroes, or bonus such as "DM's special bonus because it's saturday")
Thus a character's armor class can consist of:
10 (Base AC) + size modifier + armor + shield + dexterity modifier + deflection + natural armor + natural armor enchantment + dodge bonus + dodge bonus (...) + generic bonus + generic bonus (...)
10 (Base AC) + armor (Spell; Armor) + armor (Item; Body armor) + shield (Spell; Shield + shield (Item; Large shield)
Known Stacking of Similar Class or Racial features:
- The bard class and the harper agent class both have a special knowledge class feature (bardic knowledge & harper knowledge respectively). A character with both classes uses the total level of the two classes when calculating how effective their knowledge is.
- Damage reduction X/cold iron from the classes Spirit Shaman and Warlock stack with eachother AND with the Fey heritage feats introduced with Storm of Zehir. Damage reduction X/- from the Barbarian class and the Dwarven Defender prestige class does NOT stack however.