Glossary:Stack

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 two different items that give you a bonus of the same type or 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. 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.