A door lock looks simple from the outside because most of its work happens inside the door. A lever moves, a key turns or a fingerprint is accepted—and several mechanical parts then have to move in the correct sequence so the latch or bolt can secure the door to the frame.

Knowing the component names is useful far beyond curiosity. It makes product comparison, technical drawings, replacement orders and troubleshooting much clearer, especially when a buyer is communicating with a lock manufacturer.

First: A Lock Is a System, Not a Single Part

In everyday speech, “lock” may mean the cylinder, the handle set or the whole assembly. In hardware specifications, those parts are different. The exact construction also changes between bored/cylindrical, mortise, deadbolt and smart-lock designs.

Component Group	Function	Typical Examples
Input / credential	Tells the lock that an authorized user is requesting access.	Key cylinder, keypad, RFID reader, fingerprint sensor, app.
Operating mechanism	Transfers user input into mechanical movement.	Spindle, hub, cam, tailpiece, motor/gearbox.
Locking element	Physically holds the door closed or locked.	Latch bolt, deadbolt, hook bolt.
Door-side structure	Houses or supports the mechanism.	Lock case, mortise body, chassis, escutcheon/rose.
Frame-side structure	Receives the latch or bolt.	Strike plate, strike box, electric strike.

Cylinder

The cylinder is the key-operated input device in many mechanical locks. It contains the keyway and internal elements that permit rotation when the correct key is inserted. The cylinder itself does not necessarily hold the door shut; it usually drives another part of the lock through a cam, tailpiece or related interface.

Cylinder formats vary widely—such as key-in-knob/lever arrangements, rim cylinders, mortise cylinders and profile cylinders—so replacement requires the correct format, length, cam/tailpiece and keying requirements, not just a matching key.

Latch Bolt and Deadbolt

Latch bolt

A latch bolt is normally spring-loaded so the door can close without the user manually retracting the bolt. Its beveled face contacts the strike and pushes the latch inward; the spring then extends it into the strike opening.

Deadbolt

A deadbolt is positively moved between locked and unlocked positions rather than being pushed back by the strike during normal closing. That different operating principle is why a latch bolt and deadbolt should not be treated as the same component in drawings or specifications.

Deadlatch / Auxiliary Latch

Many locksets include a small auxiliary feature near the latch bolt. When correctly engaged by the strike, it can prevent the main latch from being pushed back in the same way it can during normal door closing. Its exact design and terminology depend on the lock architecture.

Spindle, Hub, Cam and Tailpiece

Part	What It Does	Common Confusion
Spindle	Transfers rotation from a lever/knob to the lock hub or latch mechanism.	Not every lock uses the same spindle size or shape.
Hub / follower	Receives spindle rotation and operates internal linkage.	Usually inside the lock body and not visible after installation.
Cam	A rotating profile that drives another mechanism, common in cylinders and cabinet locks.	Cam shapes and orientations are application-specific.
Tailpiece	A projecting connector that transfers cylinder/thumbturn rotation to the lock.	Length and orientation must match the receiving mechanism.

Mortise Lock Body / Lock Case

A mortise lock packages many functions inside a rectangular case installed into a pocket in the door edge. Depending on function, the case may contain the latch, deadbolt, hubs, springs, levers and cylinder interfaces. ANSI/BHMA A156.13 establishes performance requirements for mortise locks and latches in North America, including operational, strength, security, cycle, finish and dimensional criteria.

Because so much of the mechanism is integrated, mortise replacement depends heavily on dimensions: case height/width/depth, faceplate, backset, centers between handle and cylinder, spindle size, cylinder type and handing/function.

Bored / Cylindrical Lock Chassis

Bored and preassembled locks fit round openings prepared through the face and edge of a door. Their chassis, latch and trim are assembled differently from a mortise lock. BHMA terminology distinguishes these categories, which is why calling every handle lock a “mortise lock” creates procurement errors.

Rose, Escutcheon and Exterior Trim

The rose or escutcheon covers the door preparation and supports the visible operating hardware. On smart locks, the exterior trim may also carry the keypad, fingerprint reader, RFID antenna or other electronics. Appearance matters, but the trim also has to match door thickness, mounting points and internal chassis geometry.

Strike Plate and Strike Box

The strike is mounted on the frame and receives the latch or bolt. Correct strike alignment allows the bolt to enter fully without pushing the door sideways. Some installations include a strike box or reinforced structure behind the visible plate. In access-control systems, a mechanical strike may be replaced by an electric strike that releases electronically.

What Changes Inside a Smart Lock?

Smart-Lock Component	Role
Credential reader	Accepts PIN, RFID, fingerprint, mobile or other user input.
Control PCB / MCU	Processes credentials and controls the lock functions.
Motor / actuator	Converts an electronic command into mechanical locking/unlocking movement.
Position / status sensors	Detect selected states such as bolt position or door/handle events, depending on design.
Power system	Usually batteries or wired power, plus low-power management and sometimes emergency power input.
Communication module	Provides Bluetooth, Wi‑Fi or another protocol when the product supports connectivity.
Mechanical backup	May provide key or other emergency access depending on the model and project requirement.

The electronics do not eliminate mechanical fundamentals. If the mortise is misaligned or the bolt is under side load, the motor has to work harder and reliability can suffer. Smart-lock design is therefore a combination of mechanical engineering, electronics and software—not an app attached to a conventional lock.

Dimensions Buyers Commonly See on a Lock Drawing

Dimension / Term	Meaning
Backset	Distance from the door edge/faceplate reference to the operating center specified by the lock design.
Centers / PZ	Vertical relationship between handle spindle and cylinder/key center in applicable lock families.
Faceplate	Visible plate on the door edge covering the lock-body opening.
Case depth	How far the lock body extends into the door.
Door thickness range	Panel thickness the trim, fasteners and spindle/cylinder arrangement can accommodate.
Spindle size	Cross-section and dimension of the handle spindle required by the hub.

Frequently Asked Questions

Is the cylinder the same thing as the lock?

No. The cylinder is usually the key-operated input component. The complete lock includes the mechanism that actually moves the latch or bolt and the supporting hardware.

What is the difference between a latch and a deadbolt?

A latch is generally spring-loaded for automatic closing engagement; a deadbolt is positively moved into and out of the locked position.

Why do two locks that look similar require different door preparation?

The internal chassis or mortise dimensions, backset, spindle/cylinder centers, faceplate and fasteners can differ even when the exterior trim looks similar.

Which lock dimensions should a B2B buyer confirm first?

Start with door thickness, lock-body/case dimensions, backset, faceplate, spindle/cylinder centers, existing cutouts and the required lock function. Replacement projects should include a dimensioned drawing or clear photos with measurements.

Conclusion

Understanding lock anatomy turns a vague product conversation into a technical one. Instead of saying “the lock part is too short,” a buyer can identify whether the issue is the cylinder length, spindle, tailpiece, faceplate, strike or lock body. That precision saves time in specification, sampling and after-sales support.

For SRN LOCK project discussions, dimensional drawings and the required function are the best starting point. Once the mechanical architecture is clear, access methods and smart features can be selected without losing sight of the parts that actually secure the door.