By fitting a mortise lock on your door, it is difficult to force open as it is fitted into the thickness of the door. It is cut into the edge of the door and only the faceplate is visible when the door is opened. The door stile needs to be at least 45mm (1.75 inches) thick in order to take the lock.

Mortise locks are available in different sizes, but the common size is 100mm (4inches) deep. There is a narrower version available for a narrow stile, with the lock body measuring 65mm (2.5inches). If you are replacing an existing mortise lock, make sure it is the same dimensions as the previous one and the holes for the handle spindle and key are in the same positions.

The lock is operated by notches on the key engaging with levers in the lock. This makes the lock difficult to pick, as there are many levers to overcome. Although this kind of lock is difficult to pick, especially as there are such a wide variety of keys, there are other ways of forcing the lock. Use of a plastic card, similar to a credit card can be pushed through a single gap to force the bolt back. It is therefore essential when fitting this type of lock to have a snugly fitting door with a good rebate on the frame, in order to reduce the risk.

Single or double-levered locks are only intended for inside doors. This type of lock is operated by a key and can be opened from either the inside or the outside of the door. When choosing your lock, check that it conforms to British Standards and carries the Kite mark, conforming to BS 3621.
 

 
Mark a pencil line centrally on the edge of the door. Use the body of the lock as a template to draw around, keeping it central to the pencilled line. Drill out the wasted wood, using a drill bit matching the thickness of the body of the lock.

Use a bevel edged chisel to remove the rest of the wasted wood and square the edges to make the lock fit comfortably in the slot. By holding the lock against the door, mark the position of the keyhole with a bradawl. Drill a hole for the keyhole, and, by holding a spare block of wood on the other side of the door, you will stop any splintering of wood as the drill bit passes through. If applicable, using a similar method, drill another hole for the spindle of the handle.

Use a padsaw to cut out the shape for the keyhole to fit. Use an open or covered escutcheon to give a tidy finish to the keyhole. The covered escutcheon will also prevent draughts. Once the main locking mechanism has been fitted and screwed into its recess, check that it operates properly.

Push the door until almost closed and mark on the jamb the position of the striking plate. Open the door again and chisel out a recess for the body of the striking plate. Then screw the striking plate into position.
 

 
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