A mortise lock is a very strong lockset whose installation requires that it be deeply set into the wood of a door frame. It is mainly a feature of high-end homes, since it takes a degree of woodworking skill to install properly. This kind of lock is famous for its strength and security.
The mortise lock gets its name from the mortise, the hole that needs to be cut into the door in order to insert the locking mechanism. The deadbolt, locking mechanism, and knob of a mortise lock are housed in a metal box, which sometimes requires that a hole 15 inches (38.1 cm) deep be cut into the door.
The hardware and skill needed to properly set a mortise lock, means that they are a sturdy but expensive proposition for the average homeowner. To this end, in 1909 cylindrical locks were developed by a man named Walter Schlage. This type of lock required considerably less labor to install properly, needing only two bored holes.
As a result of the invention of a cylindrical lock, the demand for mortise locks have lessened. Usually, they are considered a feature of houses made before 1940 or for very luxurious houses. Less expensive houses sometimes have them only on front doors in order to add a touch of class.
For other homeowners, the cost of a mortise lock sometimes becomes irrelevant in view of increased home safety. The lock and deadbolt style of most mortise locks makes it one of the strongest locks available. The fact that it is sunk so deep into the door means it also partakes of the strength of the door itself, adding an extra layer of security.
Just because a mortise lock is strong does not mean it is foolproof. Although its complicated locking mechanism is a barrier to lock picks, it can be overcome in other indirect ways. Slipping a plastic card, such as a credit card, between the door and the jamb in order to force back the deadbolt is a common method of breaking in. This method works best on a deadbolt which doesn't fit snugly within the door.
A mortise lock should not be confused with a mortise latch. A mortise latch is very similar in appearance to a lock, with a metal box holding the mechanism which must be set into the door. The latch only has a knob, however, not a deadbolt, and does not lock.