A key which turns a series of levers operates a mortise lock. The levers will either withdraw the mortise latch to unlock the door or propel it into the housing in the latch-plate to lock the door. The mechanism of a mortise lock will have between two and seven levers. Remember that more levers will afford greater security, as the lock will be more difficult to pick. If you are fitting a mortise lock to a front door it should have at least five levers. A mortise lock can be operated from either side of the door and is often referred to as a deadlock. A deadlock should never be activated or locked from the outside when people without a key are inside the property, as it would prevent them from evacuating the building in the case of an emergency.
Another additional security measure, hinge bolts prevent the door being forced open off its hinges. Once again they are usually fitted in pairs, this time into the hinge edge of the door, one below the top hinge and one above the bottom hinge.
A sliding barrel bolt is screwed to the inside of the door. The locking bar slides along the barrel into the ring screwed to the doorframe. Front doors may well have a door bolt both at the top and bottom. Standard doors bolts come in many designs, so it is easy to find one that matches the door furniture.
A small length of chain that when in position, restricts the opening of the door to only a few inches and allows the occupant of the building to check the identity of the caller.