Automatic deadlocking
Feature on a latch which prevents the latch tongue from being pushed back whilst the door is still closed. (The credit card trick, often seen in the movies, but actually very hard to do!)

Backplate
The plate on which the moving parts of a handle or knob are mounted. 

Backset
The distance from the door edge to the centre of the keyhole or handle.

Bathroom lock
A lock fitted to an internal door which has both A) a spring latch operated by a handle or knob from either side, and B) a privacy bolt operated by a small thumb turn.

Deadlatch
A latch, either rim type or mortice type, which secures the door closed, but can be withdrawn by key from outside and by knob or handle from inside. Latch tongue cannot be pushed back (credit card trick) once the door is shut. See also NIGHTLATCH

Deadlock
A standard mortise lock, set into the edge of a door, with square-ended bolt operated by key or, occasionally, by a thumb turn on the inside.

Deadlocking
Facility in a rim lock which, using an extra turn of the key on the outside, prevents the latch or bolt from being operated when the door is closed.

Escutcheon
The surround plate for a keyhole.

Forend
The face of the lock or latch, seen in the door edge, through which the bolt or latch protrudes.

Hand
Refers to the direction of opening a door and, consequently, which way the bevelled latch must slope in order to operate.

Latch
A spring-loaded tongue which secures the door closed, but is not lockable. Certain types have now been adapted to lock as well.

Lever
Part inside a lock which is moved by the key, which allows the lock to be operated. Generally, more levers means higher security. Most insurance companies now require a 5 lever lock to be fitted to all external doors. 

Lever handle
A horizontal handle for operating the latch mechanism. The handles are usually spring-loaded to ensure return to the horizontal position after use. Some foreign handles (Not the ones we sell!) are unsprung, but this is not a problem as the latch just needs a stronger spring instead.

Nightlatch
A latch, either rim type or mortice type, which secures the door closed, but can be withdrawn by a key from outside and by a knob or handle from the inside. Often has a "snib" to hold the latch back if required. Not very secure (credit card trick!) and can simply be opened from the inside unless either deadlocked from the outside (extra turn of the key) or using a locking handle (if fitted) on the inside.

Pin tumbler lock
Type of cylinder mechanism found on most rim locks. The cylinder contains a line of small pins which, when the correct key lifts the pins to the correct position, allows the cylinder to turn. When the cylinder is rotated the lock is operated.

Rim lock or latch
A lock or latch which is fitted on the inside face/edge of a door.

Roller bolt
Found in some latches instead of a spring-loaded tongue, a bolt made with rollers to engage into the latchplate. Works with either door swing or "hand".

Rose
A cirular plate on which the moving parts of a handle or knob are mounted.

Sashlock
A mortice lock containing both A) a latch operated by handles and B) a bolt secured by key.

Spindle
The square bar inserted through a latch which connects the handles on either side of the door.

Suite
Locks can be grouped into sets that can then be operated by a controlled distribution of keys. The number of suites is virtually unlimited and can be operated under a Master Key system.