Something that prevents an attacker from clearly seeing his or her target. Concealment creates a chance that an otherwise successful attack misses (a miss chance).

Besides cover, another way to avoid attacks is to make it hard for opponents to know where you are. Concealment encompasses all circumstances where nothing physically blocks a blow or shot but where something interferes with an attacker’s accuracy. Concealment gives the subject of a successful attack a chance that the attacker missed because of the concealment.
Typically, concealment is provided by fog, smoke, a shadowy area, darkness, tall grass, foliage, or magical effects that make it difficult to pinpoint a target’s location.
To determine whether your target has concealment from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target’s square passes through a square or border that provides concealment, the target has concealment. When making a melee attack against an adjacent target, your target has concealment if his space is entirely within an effect that grants concealment (such as a cloud of smoke). When making a melee attack against a target that isn’t adjacent to you (for instance, with a reach weapon), use the rules for determining concealment from ranged attacks.
In addition, some magical effects (such as the blur and displacement spells) provide concealment against all attacks, regardless of whether any intervening concealment exists.

Concealment Miss Chance: Concealment gives the subject of a successful attack a 20% chance that the attacker missed because of the concealment. If the attacker hits, the defender must make a miss chance percentile roll to avoid being struck. (To expedite play, make both rolls at the same time). Multiple concealment conditions (such as a defender in a dog and under the effect of a blur spell) do not stack.
Concealment and Hide Checks: You can use concealment to make a Hide check. Without concealment, you usually need cover to make a Hide check.
Total Concealment: If you have line of effect to a target but not line of sight (for instance, if he is in total darkness or invisible, or if you’re blinded), he is considered to have total concealment from you. You can’t attack an opponent that has total concealment, though you can attack into a square that you think he occupies. A successful attack into a square occupied by an enemy with total concealment has a 50% miss chance (instead of the normal 20% miss chance for an opponent with concealment).
You can’t execute an attack of opportunity against an opponent with total concealment, even if you know what square or squares the opponent occupies.
Ignoring Concealment: Concealment isn’t always effective. For instance, a shadowy area or darkness doesn’t provide any concealment against an opponent with darkvision. Remember also that characters with low-light vision can see clearly for a greater distance with the same light source than other characters. A torch, for example, lets an elf see clearly for 40 feet in all directions from the torch, while a human can see clearly for only 20 feet with the same light. (Fog, smoke, foliage, and other visual obstructions work normally against characters with darkvision or low-light vision.)
Although invisibility provides total concealment, sighted opponents may still make Spot checks to notice the location of an invisible character. An invisible character gains a +20 bonus on Hide checks if moving, or a +40 bonus on Hide checks when not moving (even though opponents can’t see you, they might be able to figure out where you are from other visual clues).
Varying Degrees of Concealment: As with cover, it’s usually not worth differentiating between more degrees of concealment than described above. However, the DM may rule that certain situations provide more or less than typical concealment, and modify the miss chance accordingly. For instance, a light fog might only provide a 10% miss chance, while near-total darkness could provide a 40% miss chance (and a +10 circumstance bonus on Hide checks).

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