What is a mortise lock?
A mortise lock is a lock body installed into a rectangular pocket (mortise) cut into the edge of a door, with the operating mechanisms and bolt housed entirely within that pocket.
A mortise lock sits inside a pocket chiseled into the door edge itself, rather than mounted on the surface or fitted inside the knob. The entire lock mechanism (bolt, latch, springs, and keyhole) lives within this rectangular cavity, making mortise locks substantially deeper and more robust than modern alternatives like cylindrical locks or rim locks.
This design is the standard in older HDB flats, terrace houses, and many commercial properties across Singapore. The lock engages directly with the door frame strike plate, and the door edge bears most of the structural load. Mortise locks typically operate on a pin-tumbler cylinder and require a properly cut pocket to function correctly.
The main distinction lies in installation: mortise locks demand accurate routing into the door, whereas cylindrical locks fit into a simple hole drilled through the door thickness, and rim locks mount entirely on the door face. Mortise locks also tend to be heavier-duty, making them preferred in older buildings where durability was prioritized.
If you own an older flat or building with mortise locks, a residential locksmith can help with rekeying, repair, or replacement. The pocket depth and bolt style vary, so a locksmith will need to assess the exact model before ordering parts or upgrades.