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A master key has the capability to unlock various types of locks. Robin Smith / Getty ImagesKey Highlights
- Master keys employ a pin-and-tumbler mechanism, where a set of pins of different lengths interact with the key to lock or unlock the system. The right key aligns these pins exactly at the shear line, allowing the cylinder to rotate.
- In systems with two types of keys, a master key and a change key, the inclusion of a master wafer or spacer between certain pin pairs lets the master key work on several locks within a group, while a change key can only unlock a specific lock.
- This design’s flexibility makes it perfect for environments like apartment buildings, office complexes, and hotels, where both individual and master-level access are needed.
The superintendent of our apartment building frequently enters everyone's apartment when repairs are needed. There are probably a hundred apartments, yet he only carries one key. How is he able to access all of them with the same key?
Although locks vary greatly in appearance and features, most are based on similar principles. The most common type is the cylinder lock. This lock design works by turning a cylinder, or plug, which in turn moves an attached cam. When the plug rotates in one direction, the cam pulls on the bolt, allowing the door to open. Turning the plug the other way releases the bolt, and a spring locks it back into place to secure the door.
The key your superintendent carries is a master key. To understand how these keys function, it's essential to first grasp the basics of how locks and keys operate.
Inside a cylinder lock, there's a kind of puzzle that only the correct key can solve. The key difference among lock designs lies in the nature of this puzzle. A common type is the pin-and-tumbler design.
Turn to the next page to discover more about this lock mechanism.
Pin and Tumbler Locks
The shafts within a pin-and-tumbler lock contain numerous small pins and springs.The primary elements in a pin-and-tumbler mechanism consist of a set of tiny pins of different lengths. These pins are arranged in pairs. Each pair fits into a shaft that runs through the center of the cylinder plug and extends into the housing surrounding the plug. Springs positioned at the top of the shafts hold the pin pairs in place within the plug.
When no key is inserted, the bottom pin of each pair remains fully inside the plug, while the top pin sits halfway inside the plug and halfway in the housing. This positioning of the top pins prevents the plug from rotating — the pins bind the plug to the housing.
Inserting a key causes the notches along the key to push the pin pairs to various heights. An incorrect key will move the pins so that most of the upper pins remain partly inside the plug and partly in the housing. The correct key raises each pin pair just enough so that the point where the two pins meet aligns perfectly with the gap where the cylinder and the housing connect — this alignment is known as the shear line.
The proper arrangement of pins aligns precisely with the notches in the key.Some locks are designed to work with two separate keys. The change key unlocks only a specific lock, while the master key can open that lock and several others within the same group. These locks feature a few pin pairs separated by a third pin, known as a master wafer or spacer.
When three pins are arranged in a shaft, there are two ways the pins can be set to unlock the lock. The change key might lift the pins so the shear line is just above the master wafer’s top, while the master key might raise the pins so the shear line aligns at the bottom of the master wafer. In both instances, a gap forms at the shear line, allowing the key to turn.
In this lock design, the bottom pin has the same length across all locks in the group, while the master wafer’s length varies. This allows the holder of the master key to access any lock in the group, while someone with a change key can only unlock their specific lock.
