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Generally, when you are subject to an unusual or magical attack, you get a saving throw to avoid or reduce the effect. Like an attack roll, a saving throw is a d20 roll plus a bonus based on your class, level, and an ability score. Your saving throw modifier is: Base save bonus + ability modifier.
Saving Throw Types
The three different kinds of saving throws are Fortitude, Reflex, and Will:
These saves measure your ability to stand up to physical punishment or attacks against your vitality and health. Apply your Constitution modifier to your Fortitude saving throws.
These saves test your ability to dodge area attacks. Apply your Dexterity modifier to your Reflex saving throws.
These saves reflect your resistance to mental influence as well as many magical effects. Apply your Wisdom modifier to your Will saving throws.
Saving Throw Difficulty Class
The DC for a save is determined by the attack itself.
Automatic Failures and Successes
A natural 1 (the d20 comes up 1) on a saving throw is always a failure (and may cause damage to exposed items; see Items Surviving after a Saving Throw). A natural 20 (the d20 comes up 20) is always a success.