Mortice vs Rim Locks: What's the Difference?

On a timber door, two families of lock do nearly all the work — the mortice lock and the rim lock — and most people muddle the names. The difference is simple once you see it, and it matters, because insurers care which one you have and how it is rated. This guide explains the two, when each is used, and the single standard that decides whether yours is good enough. For the wider tour of every lock type, see our guide to door lock types; here we focus on the timber-door pair.

The basic difference

It comes down to where the lock lives. A mortice lock is fitted into the door — a pocket (the “mortice”) is cut into the edge of the door and the lock body sits inside it, hidden from view, with only the faceplate showing on the door edge. A rim lock is mounted onto the surface — it sits on the inside face (the “rim”) of the door, screwed to the back. Concealed versus surface-mounted: that is the whole distinction.

Mortice locks

Because the body is housed inside the door, a mortice lock is neat and strong, and it is the traditional choice for a timber front door. There are two main kinds. A mortice deadlock is key-operated only — it throws a solid bolt and has no handle or latch, so it is the secure “lock it and leave it” bolt. A mortice sashlock combines that deadbolt with a spring latch worked by handles, so one unit both latches and locks — common on back and internal doors. For security, the number that matters is the lever count: a five-lever mortice lock is the one to have. The lever principle itself is older than you might think — it dates to Robert Barron’s 1778 patent, the story of which we tell in our history of locksmithing.

Rim locks

The rim family is led by the nightlatch — the classic “Yale-type” lock that latches automatically as the door closes, opened by a key outside and a knob inside. It is convenient and easy to fit, which is why so many doors have one. But a basic nightlatch on its own is not enough to satisfy most insurers: many can be slipped, and a standard one does not deadlock or need a key from inside. A deadlocking nightlatch, or better a rim lock built to British Standard, is a different proposition — and a rim deadlock gives a surface-mounted bolt where cutting a mortice is not practical.

Which do you need?

For a timber front door, the answer is usually both, fitted at different heights: the nightlatch up top for convenience, the mortice deadlock below for security. On back doors a sashlock often does the job; on internal or low-risk doors you can keep it simple. The decision is rarely “mortice or rim” in the abstract — it is making sure the door has at least one lock that meets the standard your insurer expects.

The standard that decides it

That standard is BS3621 — the British Standard for thief-resistant locks on final exit doors, marked with the BSI Kitemark and operated by a key from both sides. It applies to both mortice locks and rim nightlatches: a lock is only BS3621 if it is built and tested to it, so an ordinary nightlatch or a non-rated five-lever does not count. We decode that mark and all the others in our guide to lock security grades and Kitemarks, and cover the standard in depth in British Standard locks explained.

Where we come in

If you are not sure whether your timber door is properly protected — or whether that old nightlatch would satisfy a claim — the quickest answer is to have someone qualified look. Every LocksmithLocal locksmith can identify what you have, fit or upgrade mortice and rim locks to BS3621, and set the door up the way an insurer expects. Find your local locksmith to arrange it.