The modern Yale 89 nightlatch door lock can be operated by lever internally or also by key externally. The inside handle can be deadlocked by a reverse turn of the key in the outside cylinder after closing the door. This prevents the lock from being “slipped” or “bypassed” from the exterior without a key.
Internally the Yale 89 deadlocking nightlatch 60 mm can be “Snibbed” to deadlock or hold back the latch for convenience.
Other product variations include:
Yale 89 Deadlocking Nightlatch – Nightlatch (P89S)