dead bolt

right|thumb|Door with two locks, one in the doorknob and a separate deadbolt A deadbolt or deadlock is a type of lock morticed into a wooden door where a bolt is thrown into the door frame, using a key from either side, to secure the door. It is distinct from a spring bolt lock because a deadbolt can only be opened by a key or handle. 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, as well as make the door more resistant to forced entry. A deadbolt is often used to complement a spring-bolt lock on an entry door to a building.

== Common types == A deadlock, if it is cylinder operated, 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 (and often do not have) any twist knob. This prevents unwanted unlocking of the door by forced access to the interior twist knob (via a nearby window, for example). Double cylinder locks are sometimes banned from areas because they can be difficult to open from the inside and violate fire safety regulations. Some lock manufacturers also have a "lockable" knob: a key is always needed on one side (usually external), and a twist knob can be used on the other (internal), unless a button has been pressed, in which case a key is also needed on the internal side.