Deadbolt

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.

In the event of a fire, occupants will be prevented from escaping through double-cylinder locked doors unless the correct key is used.

[citation needed][4] The risk can be mitigated by locking the deadlock only when there are no occupants inside the building, or leaving the key near the keyhole.

Note that single cylinder dead locks (with an unlocked twist mechanism on the inside of the door) do not have this problem, and therefore are most commonly used on fire exits.