A dead bolt is a locking mechanism where it cannot be moved to the open position except by rotating the lock cylinder with the key. The more common spring bolt lock uses a spring to hold the bolt in place, allowing retraction by applying force to the bolt itself. A deadbolt can therefore make a door more resistant to entry without the correct key rather than a spring bolt. A deadbolt is often used to complement a spring-bolt lock on an entry door to a building.
A deadlock may be either single cylinder or double cylinder. A single cylinder deadlock will accept a key on one side of the lock, but is operated by a twist knob on the other side. Double cylinder locks will accept a key on both sides and therefore do not require any twist knob. This prevents unwanted unlocking of the door by forced access to the interior twist knob. A variant of the standard deadbolt is the vertical deadbolt. They resist jimmying when an intruder inserts a pry bar between the door and the jamb and attempts to pry the bolt out of the door.