What Are the Parts of a Door Lock?

Most people don't think of door locks as anything special. Because we use them regularly — when we go to the bathroom, when we leave home for the day, when we get in and out of our cars — they're often considered mundane and ignored. However, the lock hidden inside of the average door is much more complex than you might expect.

Though there are slight differences depending on the lock's manufacturer, there are three primary parts of a door lock to keep in mind when learning about the way they work. Understanding the function of each part can help you troubleshoot problems with a lock on your own, which might save you a call to your local locksmith if trouble comes around.

Most homes, small commercial buildings and multipurpose spaces like schools and libraries still use traditional key-and-knob door locks, formally known as ​mechanical locks.​ However, mechanical locks aren't the only type of lock available. Various forms of ​electronic locks —​ which use electricity and small motorized components instead of or alongside traditional mechanical lock parts — have become popular and have become affordable enough to be adopted by a number of homeowners and business owners.

In recent years, the widespread adoption of smart technology has made these electronic locks more advanced. Today, ​a smartphone or a voice sample can open a door​ as easily as any key. Despite being more technologically advanced, however, the basic anatomy of electronic locks and mechanical locks are incredibly similar.

Every door lock has both internal and external components. Door handle or door knob parts — in addition to the keypad or scanner on an electronic lock — can be considered external, while the internal parts are composed of everything hidden inside the door and everything behind the key slot. The internal components can be considered the lock itself. These are the parts of the lock assembly that secure the door and allow it to be opened by using the appropriate key, while the external components are used to operate the door or interact with the locking mechanisms.

While the shape and style of the external components can vary depending on the type of door knob or handle or whether you unlock the door with a traditional metal key, your fingerprint or a card, ​the internal components will always involve a spring latch or deadbolt,​ secured with a box and strike plate. These latches or bolts are extended or retracted by the mechanisms in the lock's body.

The lock body, or lock cylinder, is the core of a door lock. The lock body turns to engage or disengage the lock's bolt or latch when the appropriate key is used. In a mechanical lock, the lock body uses a series of spring-loaded pins to allow or stop the turning. The combination of pins that need to be pushed to turn the lock is unique to each lock body and matches the grooves of the door's key. When the key is inserted, it allows the lock body to turn.

In an electronic lock, small motorized components controlled by the keypad, scanner or wireless signal reader control the bolt or latch — sometimes alongside a mechanical lock's pins and sometimes replacing the pin system entirely. When the appropriate code or key signal is read, an electronic signal engages the bolt or latch by way of a motor. This is more secure than a mechanical lock, but it is also more expensive to maintain.

Connected to and controlled by the lock body, the latch or deadbolt is the part of the door lock that holds the door in place when it's locked. When the lock body turns, the latch or deadbolt moves to either allow or stop the door from being opened.

Generally, door locks will either use a traditional ​deadbolt​ — a piece of metal that extends and holds the door shut when the lock body is turned — or a ​spring latch,​ a bolt that uses a spring clip to automatically lock the door whenever it closes. While these sound similar, there is a difference. A lock with a spring latch is locked by default, whereas a lock with a deadbolt is ​unlocked​ by default.

When a door lock is used, it moves the latch or deadbolt, but there needs to be a space for these components to go. The space inside the door's frame and the metal plate used to reinforce that space create a secure location for the bolt or latch. Known as the ​box and strike plate,​ this final lock component guides and secures the lock's bolt to keep the door secure and shut and prevents the locked door from simply being ripped open.