What Is a Mortice Lock? A Complete Guide to Types and Sizes

At Trade Door Handles, we've been specialist suppliers going back decades, stocking a large range of locks and latches for doors of every kind — including bathroom locks, 2, 3 and 5 lever locks, sash locks, deadlocks, tubular latches and more for commercial and domestic properties. Of all the terms in our architectural ironmongery glossary, "mortice lock" is the one we're asked about most, simply because it covers such a wide family of products.

This guide explains exactly what a mortice lock is, the different types available, which doors they suit, standard sizes, and whether fitting one is a DIY job or a task for a joiner.

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A mortice lock is a lock that's fitted into a pocket cut into the edge of a door, rather than being mounted on the surface. Once installed, only the faceplate is visible on the door edge, with the keyhole or handle fittings showing on the front and back faces. Because the lock body sits inside the door itself, mortice locks are widely regarded as stronger and neater than surface-mounted alternatives, which is why they remain the standard choice for timber doors across the UK.

"Mortice" (sometimes spelled "mortise") is a carpentry term for a pocket, hole or recess cut into a piece of timber to receive another component. In door hardware, the mortice is the rectangular pocket chiselled into the door edge to house the lock body — the same term used in traditional mortice and tenon joinery. A mortice lock simply takes its name from this method of fitting, as opposed to a rim lock, which sits on the surface rather than inside the door.

Mortice locks aren't a single product — the term covers several distinct lock formats, each suited to different doors and levels of security:

• Mortice sash locks — combine a sprung latch and a key-operated deadbolt in one case, and need handles fitted on both sides of the door.
• Mortice deadlocks — contain only a key-operated deadbolt, with no latch and no need for handles.
• Mortice latches — a sprung latch with no locking function at all, simply holding the door shut.
• DIN mortice locks — built to the DIN (Deutsches Institut für Normung) European standard rather than the more common British sizing, typically used on continental-style doors or specific commercial specifications.

Take a look at our full range of Mortice Locks and Latches, including our dedicated Mortice Latches and DIN Locks collections, if you already know which format you need.

A mortice sash lock includes both a latch and a deadbolt, so the door closes and holds itself shut automatically while still offering key security when needed. A mortice deadlock has no latch at all — it's operated entirely by key from both sides, which makes it common as a secondary lock rather than the primary one. If you're fitting a cylinder-operated version of either type rather than a traditional lever key lock, our Euro Profile Sash Locks collection uses the same mortice fitting method with a replaceable cylinder instead.

The core difference is where the lock sits. A mortice lock is recessed into a pocket inside the door edge, while a rim lock is fixed to the surface of the door, with only the keep or staple cut into the frame. Rim locks are often chosen for older or thinner doors where cutting a full mortice pocket isn't practical, or where a traditional surface-mounted look is preferred. Our Rim Locks and Rim Latches collection covers this alternative if a mortice lock isn't the right fit for your door.

Mortice locks are designed for timber doors, since they rely on a mortice pocket being cut into solid wood. Typical locations include:

• Front and back entrance doors — usually a 5 lever sash lock or deadlock for insurance-rated security.
• Internal bedroom and study doors — often a 2 or 3 lever sash lock or a simple mortice latch.
• Bathroom doors — more likely to use a privacy-turn lock than a full sash lock or deadlock, covered in our Bathroom Locks and Deadbolts collection.

Composite and uPVC doors typically use a different locking system entirely — usually a multipoint mechanism operated by a euro cylinder — so a traditional mortice lock isn't generally an option on those door types.

Fitting a mortice lock is a realistic DIY job for anyone comfortable using a chisel and drill carefully, since it comes down to patience rather than specialist skill for most standard timber doors. We've written a full step-by-step walkthrough in our guide on how to fit a mortice lock, covering everything from marking the position to fitting the keep. That said, a professional joiner or locksmith is worth calling in for fire doors, listed buildings, or anywhere the door's certification needs to remain valid, since incorrect fitting can affect both.

It depends on the type of mortice lock you're fitting. A mortice sash lock or mortice latch needs handles fitted on both faces of the door, since the latch is operated by turning the handle. A mortice deadlock doesn't need handles at all, as it's operated purely by key. If you're specifying a full set, our Mortice Key Locks collection is a good starting point for matching the lock to the right handle set.

Mortice lock sizing is described by the case size, which relates to the backset — the distance from the edge of the door to the centre of the keyhole. In the UK, the most common case sizes are 64mm (2.5") and 76mm (3"), though 3 lever and 5 lever versions are also available in a range of larger sizes for wider doors or specific handle centre requirements.

The 64mm (2.5") case size remains the most widely fitted mortice lock size on standard UK timber doors, offering a practical balance between handle position and lock strength.

Fire doors and doors with deep casing or architrave often call for the larger 76mm (3") case size instead, since the deeper backset moves the handle further away from the edge of the door. Without that extra distance, a handle fitted with a standard 64mm lock can sit too close to the casing, making it awkward to grip properly or catching against the frame — much the same underlying issue as the horizontal mortice locks covered above, just solved here with a deeper case rather than a horizontal spindle. If your door has a particularly wide architrave, or is a certified fire door, it's always worth confirming the required backset with your door manufacturer or joiner before ordering.

Always measure your existing lock's backset and case size before ordering a replacement, to avoid needing to re-cut the mortice pocket.

A horizontal mortice lock has its keyhole and spindle positioned side by side on the same horizontal plane, rather than the keyhole sitting below the spindle as on most modern locks. This places the key and handle at the same height, giving panelled doors a more spacious, symmetrical look.

Horizontal mortice sash locks are almost always paired with mortice knobs rather than lever handles, since a knob sits centrally on the spindle rather than reaching outward the way a lever does. Positioning the spindle further along the lock case moves the knob further away from the door's edge, and in turn further from the door casing or frame — which is the whole point of the design, since it stops your knuckles catching on the frame every time you turn the knob to open the door. Our Mortice Knobs collection is designed specifically to pair with this type of mortice lock.

Horizontal mortice locks were especially common in Georgian and Victorian properties, before lever handles became standard, and they're still widely specified today when restoring or matching hardware in period homes.