A mortice lock is 'morticed' into the door. If you are replacing a mortice lock, it is important that you know the size of the your existing lock to ensure that your new lock will fit into the same hole. Mortice locks are available in sashlock or deadlock variants, so it is important that you know which one you require.
Measuring the backset
The measurement from the front plate of the lock to the back of the lock case. A number of different backsets are available but the most common in the UK are 64mm (2.5") or 76mm (3").
Measuring the centres
If you are fitting a sashlock to enable the lock to be operated in conjunction with a door handle, you will need to measure from centre of the 'follower' - the hole where the handle spindle goes, to the centre of the keyhole.
Typically in the UK, the most common size is 57mm, although other sizes are available,
Sashlock or Deadlock?
A sashlock is typically used on back doors and has a latch and a bolt. The latch is operated by a handle and the bolt is operated by key.
A deadlock is typically used on front doors in conjunction with a nightlatch and has a bolt only. The bolt is operated by key.
How many levers?
A mortice lock is either operated by a euro cylinder or by integral levers, The more levers a morice lock has, the more secure it is. Look for a British Standard 5 lever mortice lock for all external timber doors.