Armor Class (AC)
Your Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on you. It's the attack roll result that an opponent needs to achieve to hit you.
Your AC is equal to the following:
10 + armor bonus + shield bonus + Dexterity modifier + other modifiers
Note that armor limits your Dexterity bonus, so if you're wearing armor, you might not be able to apply your whole Dexterity bonus to your AC (see Table: Armor and Shields).
Some attacks completely disregard armor, including shields and natural armor—the aggressor need only touch a foe for such an attack to take full effect. In these cases, the attacker makes a touch attack roll (either ranged or melee). When you are the target of a touch attack, your AC doesn't include any armor bonus, shield bonus, or natural armor bonus. All other modifiers, such as your size modifier, Dexterity modifier, and deflection bonus (if any) apply normally. Some creatures have the ability to make incorporeal touch attacks. These attacks bypass solid objects, such as armor and shields, by passing through them. Incorporeal touch attacks work similarly to normal touch attacks except that they also ignore cover bonuses. Incorporeal touch attacks do not ignore armor bonuses granted by force effects, such as mage armor and bracers of armor.
Flat Footed AC
Sometimes you can't use your Dexterity bonus (if you have one). If you can't react to a blow, you can't use your Dexterity bonus to AC. If you don't have a Dexterity bonus, your AC does not change.
Base Attack Bonus (BAB)
Each creature has a base attack bonus and it represents its skill in combat. As a character gains levels or Hit Dice, his base attack bonus improves. When a creature's base attack bonus reaches +6, +11, or +16, he receives an additional attack in combat when he takes a full-attack action (which is one type of full-round action—see Combat).
A spell's power often depends on its caster level, which for most spellcasting characters is equal to her class level in the class she's using to cast the spell.
In the event that a class feature or other special ability provides an adjustment to your caster level, that adjustment applies not only to effects based on caster level (such as range, duration, and damage dealt), but also to your caster level check to overcome your target's spell resistance and to the caster level used in dispel checks (both the dispel check and the DC of the check).
Combat Maneuver Bonus
Each character and creature has a Combat Maneuver Bonus (or CMB) that represents its skill at performing combat maneuvers (see more in chapter “How Combat Works”). A creature's CMB is determined using the following formula:
CMB = Base attack bonus + Strength modifier + special size modifier
Creatures that are size Tiny or smaller use their Dexterity modifier in place of their Strength modifier to determine their CMB. The special size modifier for a creature's Combat Maneuver Bonus is as follows:
- Fine –8
- Diminutive –4
- Tiny –2
- Small –1
- Medium +0
- Large +1
- Huge +2
- Gargantuan +4
- Colossal +8
Some feats and abilities grant a bonus to your CMB when performing specific maneuvers.
To cast a spell, you must concentrate. If something interrupts your concentration while you're casting, you must make a concentration check or lose the spell. When you make a concentration check, you roll d20 and add your caster level and the ability score modifier used to determine bonus spells of the same type. Clerics, druids, and rangers add their Wisdom modifier. Bards, paladins, and sorcerers add their Charisma modifier. Finally, wizards add their Intelligence modifier. The more distracting the interruption and the higher the level of the spell you are trying to cast, the higher the DC. If you fail the check, you lose the spell just as if you had cast it to no effect.
Damage Reduction (DR)
Some magic creatures have the supernatural ability to instantly heal damage from weapons or ignore blows altogether as though they were invulnerable. Sometimes damage reduction represents instant healing. Sometimes it represents the creature's tough hide or body. In either case, other characters can see that conventional attacks won't work.
The numerical part of a creature's damage reduction (or DR) is the amount of damage the creature ignores from normal attacks. Usually, a certain type of weapon can overcome this reduction (see Overcoming DR). This information is separated from the damage reduction number by a slash. For example, DR 5/magic means that a creature takes 5 less points of
damage from all weapons that are not magic. If a dash follows the slash, then the damage reduction is effective against any attack that does not ignore damage reduction.
Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury poison, a monk's stunning, and injury-based disease. Damage Reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact. Attacks that deal no damage because of the target's damage reduction do not disrupt spells.
Spells, spell-like abilities, and energy attacks (even non-magical fire) ignore damage reduction. If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.
Fortitude saves measure your ability to stand up to physical punishment or attacks against your vitality and health.
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 and level, and an associated ability score. Apply your Constitution modifier to your Fortitude saving throws.
Reflex saves test your ability to dodge area attacks and unexpected situations.
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 and level, and an associated ability score. Apply your Dexterity modifier to your Reflex saving throws.
Will saves reflect your resistance to mental influence as well as many magical effects.
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 and level, and an associated ability score. Apply your Wisdom modifier to your Will saving throws.