The Core Mechanic
Whenever you attempt an action that has some chance of failure, you roll a twenty-sided die (d20). To determine if your character succeeds at a task you do this:
- Roll a d20.
- Add any relevant modifiers.
- Compare the result to a target number.
If the result equals or exceeds the target number, your character succeeds. If the result is lower than the target number, you fail.
Dice rolls are described with expressions such as “3d4+3,” which means “roll three four-sided dice and add 3” (resulting in a number between 6 and 15). The first number tells you how many dice to roll (adding the results together). The number immediately after the “d” tells you the type of die to use. Any number after that indicates a quantity that is added or subtracted from the result.
Percentile dice work a little differently. You generate a number between 1 and 100 by rolling two different ten-sided dice. One (designated before you roll) is the tens digit. The other is the ones digit. Two 0s represent 100.
A modifier is any bonus or penalty applying to a die roll. A positive modifier is a bonus, and a negative modifier is a penalty.
In most cases, modifiers to a given check or roll stack (combine for a cumulative effect) if they come from different sources and have different types (or no type at all), but do not stack if they have the same type 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 and worst penalty applies. Dodge bonuses and circumstance bonuses however, do stack with one another unless otherwise specified.
The bonus or penalty associated with a particular ability score. Ability modifiers apply to die rolls for character actions involving the corresponding abilities.
An armor bonus applies to Armor Class and is granted by armor or by a spell or magical effect that mimics armor. Armor bonuses stack with all other bonuses to Armor Class (even with natural armor bonuses) except other armor bonuses. An armor bonus doesn’t apply against touch attacks, except for armor bonuses granted by force effects (such as the mage armor spell).
A circumstance bonus (or penalty) arises from specific conditional factors impacting the success of the task at hand. Circumstance bonuses stack with all other bonuses, including other circumstance bonuses, unless they arise from essentially the same source.