Deadbolts are normally used on exterior residential doors. They offer much greater security against physical attack than an entry lock, though both should be used. With most deadbolts, the bolt extends a full inch. In most cases, if the bolt does not fully extend, it doesn’t actually lock in position. The deadbolt strike should be mounted to the door frame in a manner that secures it to the structural framing in the wall around the door in order to provide the greatest resistance to physical attack. Deadbolts are offered as single or double cylinder versions. A double cylinder deadbolt requires the use of a key from both the inside and outside to either extend or withdraw the bolt. Fire codes may restrict the use of double cylinder deadbolts in residential settings. Deadbolts are generally not allowed in commercial settings except sometimes on the primary entrances with the provision that the deadbolt is not to be engaged while the building is occupied or open to the public.