A nightlatch is mounted on to the door instead of being morticed into it. The size of nightlatch you require is often dependent on whether or not you are fitting it to a panelled door.
Measuring the backset
Nightlatches are available in two different sizes - 40mm or 60mm. This measurement is the distance from the keyhole to the edge of the door. If you have a panelled door you will probably need to buy a 40mm nightlatch to make sure that it fits on to the door.
Double locking nightlatches
Double locking nightlatches have a keyhole on the handle inside the door so that the nightlatch can be deadlocked from inside. This is useful if you have a glass panelled door as if a burglar smashes the glass and reaches in, they won't be able to unlock the door.
There are many types of nightlatches available. If you are securing an external door, always look for a British Standard nightlatch to ensure a high level of security.