The knock spell enables stuck, barred, locked, held, or arcane locked doors to be opened, unlocking them or removing impediments to opening. It unlocks secret doors and locked chests, and defeats the locking mechanisms of trick-opening boxes. It also loosens shackles or chains (provided they serve to hold closures shut).

When you complete the casting of the spell, make two caster level checks, with an additional bonus equal to 10 + your Intelligence bonus. The result of each check is compared to the DC of the simplest means of securement on the portal or container (i.e., the one with the lowest DC to bypass it); if the check is successful, that means of securement is unlocked (or otherwise placed in an open state). Resolve these checks in sequence (i.e., the result of the first check is determined first, then the second one). (Thus, since a single casting of knock can undo at most two means of securement, if a door is locked, barred, and held, or quadruple locked, opening it requires at least two knock spells.)

If the portal or container is held or arcane locked, the hold portal or arcane lock effect is always considered to be a simpler means of securement than any non-magical locking devices, no matter what the DCs of any such mechanisms might be; thus, the knock spell always attempts to defeat a hold portal or arcane lock effect first. The effective DC of an arcane lock effect, for the purposes of the knock spell, is equal to 20 + the caster level of the arcane lock. A hold portal effect is defeated automatically (i.e., its effective DC against knock is 0).

If used to defeat an arcane lock effect, the knock spell does not remove the arcane lock but simply suspends its functioning for 10 minutes. In all other cases, the defeated securement mechanism does not relock itself, become stuck again, or otherwise re-engage on its own.

Knock does not raise barred gates or similar impediments (such as a portcullis), nor does it affect ropes, vines, and the like. The effect is limited by the area. A 3rd-level caster can cast a knock spell on a door of 30 square feet or less (for example, a 4-foot-by-7-foot door).