Rim, Mortise and Cylinder Basics

Three words that get mixed up

Ask ten people what a lock is and you will hear ten answers. Some mean the key and keyhole. Some mean the knob. Some mean the heavy bolt that slides into the frame. In the trade, these are separate pieces with separate names, and the names matter because the parts are bought, repaired and replaced separately.

A cylinder is the part that accepts a key, the small round or oval component with the keyway. A lock body, or case, is the housing that holds the bolt or latch and the mechanism that moves it. Rim and mortise describe how that body is mounted: on the surface of the door, or inside a pocket cut into its edge.

Once you know that, many puzzling conversations clear up. A locksmith who says the cylinder is worn but the case is fine is telling you that only a small part needs replacing. A locksmith who says the mortise case is cracked is telling you something larger. Understanding the vocabulary also helps you describe your own door when you call.

This page stays at the level of everyday ownership. It does not explain how to defeat anything, and it avoids detailed repair steps that belong to a trained professional.

The cylinder: the part that listens to the key

The cylinder is the heart of most key operated locks. Inside it, a set of small parts reacts to the cuts on your key. When the key is right, the plug can turn. When it is wrong, the plug stays put. Our page on how a pin tumbler lock works in plain words explains the common design.

Cylinders come in several shapes. A rim cylinder is a small round cylinder that passes through the door and connects to a rim lock. A mortise cylinder is a threaded, often oval or round piece that screws into a mortise case. A profile cylinder, common in other parts of the world, has an oblong outline. A cylinder in a typical house deadbolt or knob lock is often a simple round piece built into the lock.

The cylinder is also the part you change when you rekey. A locksmith may swap the pins inside, or replace the whole cylinder with one that takes a different key. Either way, the case and the door hardware can stay. See our page on rekeying for how this works in practice.

Because a cylinder is a self contained unit, it can sometimes be replaced independently of the rest of the lock. That is useful when a cylinder wears out, when a key breaks and damages it, or when you want a different keyway.

Rim locks: mounted on the surface

A rim lock sits on the inside face of a door. Its case is a box screwed to the surface, and the bolt extends toward the frame, where a matching keeper receives it. The cylinder passes through a hole in the door, connecting the outside key to the case on the inside.

Rim locks are found on older doors, on some apartment entries, on gates and on certain secondary doors. They are simple to mount because they require little cutting of the door. Some rim locks include a spring latch that catches automatically when the door closes, and a deadbolt feature that you throw by hand.

A rim lock often depends on the door being thick enough and the surface being flat. A door with a decorative panel or a rounded face may need adjustments. Because the case is visible, the lock also has a distinct look, and it can be changed without major carpentry.

If a rim lock stops working, the cause may be the cylinder, the case, the keeper, or the door alignment. A locksmith can usually tell by operating the lock with the door open and then closed. Our guide on door thickness and lock fit notes why door dimensions matter.

Apartments and condos add a wrinkle. Many have rim or surface mounted deadbolts as a second lock, added over the years by tenants or management, along with a cylindrical lock in the handle. If you rent, ask the property manager what you are allowed to change before you start, because requirements vary. Our page on apartment lockout information covers the building side.

Gates and sheds often use rim style hardware or padlocks, which add their own care needs in damp weather. A little attention to rust and a drop of the right lubricant keep them turning. Our guide on padlocks for gates and sheds goes further.

Mortise locks: set into the edge of the door

A mortise lock is installed inside a deep pocket cut into the edge of the door. The case holds a latch, a deadbolt and the mechanism that links them to the handle and key. Because the case sits within the door, you see only the faceplate on the edge, the handle on each side and the cylinder in the face.

Mortise locks are known for their sturdiness. They are common in older homes, in many commercial buildings and in some high end residential doors. Their heavy construction and the solid wood around the pocket can give a firm feel. Our page on mortise locks in older homes discusses what homeowners often find.

The cylinder in a mortise lock is a separate piece that screws into the case from the face of the door. It is held by a small set screw. That separation is a big advantage, because a worn or damaged cylinder can be replaced without replacing the whole lock. It also means the cylinder can be changed to suit a master key system.

Mortise cases are not all alike. Dimensions, spindle positions and trim styles vary, and finding a matching replacement can take care. Because the pocket is cut into the door, a replacement case has to fit the existing pocket, or the door must be modified. That is a job for someone who has done it before.

• A cylinder takes the key and can often be replaced on its own.
• A rim lock is mounted on the inside surface of the door.
• A mortise lock sits in a pocket cut into the door edge.
• A cylindrical lock, like most knob and lever sets, uses a bored hole through the door face.

Cylindrical locks: the type most homes have

Most modern houses use cylindrical locks. A hole is bored through the face of the door for the main lock, and a smaller hole is bored into the edge for the latch. The knob or lever holds the cylinder, and a latch bolt extends from the edge. A deadbolt is usually a separate unit installed in its own bored hole above the handle.

This design is economical and easy to install, and it is why standard locksets are widely available. Replacement parts are common, and the hole sizes follow familiar standards. Our page on backset and door prep measurements describes the numbers that decide fit.

A cylindrical lock is not necessarily weak. A good one on a solid door with a strong strike can serve for decades. But it has fewer moving parts than a mortise lock, and when it fails the usual remedy is replacement of the lockset. Our guide on choosing between knob, lever and handleset locks compares the styles.

Do not confuse cylindrical with cylinder. A cylindrical lockset contains a cylinder, but the word describes the way the lock is mounted in the door.

Why the difference matters when something breaks

The type of lock decides what repair is realistic. If a cylinder in a mortise lock is worn, replacing it is a modest job. If the case itself is broken, you may need a replacement case that fits the existing pocket, which can be harder. If a rim lock has a cracked case, a new rim lock can be mounted on the same door surface with relatively little change.

Cylindrical locks tend to be replaced as a unit. If the latch sticks or the knob wobbles, a new lockset is often more sensible than a repair. A locksmith will help you decide, and will explain the work and the price before starting.

Age also matters. Older mortise and rim locks may use parts that are no longer made. A skilled professional can sometimes repair or rebuild them, or fit a modern equivalent. If the lock is part of the character of an older home, ask about restoring it instead of replacing it. Our guide on skeleton keys in historic homes explains related issues.

When you call, describe the lock as well as you can. Say whether the lock sits on the surface or in the edge of the door, whether you see a cylinder in the face, and how many keyholes you can see. That alone helps narrow the possibilities.

Think about the trim as well. The handle, the rose and the escutcheon plate that surround a lock are often decorative and separate from the working parts. Replacing worn trim can freshen the look of an entry without touching the mechanism, and some trim is held on by screws you can see. If you want to keep the character of an older door, ask whether the trim can stay while the inside is updated.

Cost and effort usually rise as you move from a cylinder to a case to a full lock on a modified door. That is another reason to identify the type before you begin, since the least costly sensible fix is often the smallest part that explains the problem.

Keys, keyways and compatibility between types

A key fits a cylinder, not a lock type. This means that rim, mortise and cylindrical locks can all use the same kind of key if their cylinders share a keyway. A locksmith can sometimes match several locks of different types to one key by fitting compatible cylinders.

This is useful in a home with a mixture of doors. An older front door with a mortise lock and a newer back door with a deadbolt can often be keyed alike. The process depends on the available cylinders and keyways, and requirements differ by brand. Our glossary page on keyway and bitting explains the terms.

Do not assume that a key that fits the front door will fit a neighbor's lock of the same type. Keyways may look alike but differ in detail, and the cuts will differ even more.

If you want a single key for several doors, ask whether the existing cylinders can be rekeyed or whether new ones are needed. Our guide on lock replacement or rekeying helps you weigh the options.

Doors, climate and the life of the hardware

Central Florida is tough on hardware. Humid air, rain, salt on coastal breezes and strong sun can corrode, swell and dull the parts. A mortise lock inside a wooden door can be affected when the wood swells in humid weather, tightening the fit. A rim lock exposed to rain may rust at its screws.

Gentle maintenance goes a long way. Keep the faceplate and the strike clean, use the right lubricant on moving parts, and watch for signs of stiffness. Our guide on lock maintenance for hot climates offers a routine.

Doors move with the seasons. If a lock begins to bind each summer, the cause may be the door rather than the lock. Our article on humidity and swollen doors describes what happens.

After a storm, check that locks still operate smoothly and that nothing has shifted. A door that has been soaked and dried can change shape slightly, and a lock that worked yesterday may bind today.

What to expect from a locksmith visit

When you call, a locksmith will ask what is happening and where. They will check that you have authority over the door before changing or rekeying, explain the work and the price before starting, and test the finished lock with you. For unusual hardware, they may ask for a photo or a few measurements.

If you have a mortise lock, they may ask for the brand on the faceplate and the shape of the faceplate itself. If you have a rim lock, they may ask how the case is mounted and whether there is a deadbolt feature. If you have a cylindrical lockset, they may ask for the backset and the door thickness.

Be ready to say whether you want to keep the existing lock, restore it, or replace it. If you are not sure, say so. A locksmith can walk through the choices without pressure. See lock replacement and repair for the kinds of work involved.

After the work, try the key several times from both sides, operate the latch and the deadbolt, and close the door gently and firmly. If anything feels wrong, say so before the visit ends.

A short picture guide to describe your own door

Walk up to your door and look at it as if you were describing it to a stranger. Is there a box mounted on the inside face, or is the hardware flush? Is there a long thin plate along the edge, or just a small latch? How many keyholes do you see, and how big are they? Does the handle sit on a long plate or a round rose?

Write down the answers, and take two photos: one of the inside face and one of the edge with the door open. Those two photos tell a locksmith a great deal. Add the approximate age of the home if you know it.

With that information, the conversation moves straight to the useful questions: what is worth keeping, what needs replacing, and what the options cost. You will also be more confident when someone explains the work to you.

Finally, remember that you do not need to master all of this to make a good decision. You only need to know the right questions: what part is failing, what does it cost to fix that part, and what would a full replacement involve? A locksmith who can answer those clearly, before work begins, is giving you what you need.

Questions people ask