The knock spell opens stuck, barred, locked, held, or arcane
locked doors. It opens secret doors, as well as locked or
trick-opening boxes or chests. It also loosens welds, shackles,
or chains (provided they serve to hold closures shut). If used to
open a arcane locked door, the spell does not remove the arcane
lock but simply suspends its functioning for 10 minutes. In all
other cases, the door does not relock itself or becomes stuck
again 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. Note that 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 standard 4-foot-by-7-foot door).
Each spell can undo up to two means of preventing egress through
a portal. Thus if a door is locked, barred, and held, or
quadruple locked, opening it requires two knock spells.