Most manufacturers make both deadlocking and non deadlocking nightlatches,what's the difference and when should you use?

The difference?
 
Deadlocking nightlatches such as the Yale 85 can be locked from the outside by the key meaning the the handle on the inside will be locked and it will not retract the latch,this can also be known as double locking.

Non deadlocking nightlatches such as the Yale 91 by contrast cannot be double locked.

When to use a deadlocking nightlatch?
Once a nightlatch has bee...
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