A skeleton key (also known as a passkey) is a type of master key in which the serrated edge has been filed down so that it can open numerous locks, most commonly the warded lock. The term derives from the fact that the key has been reduced to its essential parts.
In a broader sense the term can be used synonymously with master key to refer to any key, keycard or other device capable of opening a variety of locks.
A skeleton key is a key that has been filed or cut to create a master that can be used to unlock a variety of warded locks each with a different configuration of wards. This can usually be done by removing most of the centre of the key, allowing it to pass by the wards without interference, operating the lock. To counteract the illicit creation of such keys, locksmiths can put wards not just in the centre but on the outside as well, making the creation of a skeleton key more difficult.
Lever lock skeleton keys are used in a lock with usually three or five levers and a set of wards that come into contact with the bit of the key only on the sides – the top is for pushing the levers to their correct heights while the warded section of the key just has to pass uninterrupted to allow the key to rotate fully. A master key system of lever locks has the same lever heights in all locks. Each door will have different wards and can only be opened by the correctly warded key or the master key. A skeleton key has the warded section of the key removed so that it opens all the doors of a system. Some applications, such as a building with multiple entrance doors, have numerous locks that are keyed alike; one key will open every door. A keyed-alike system is different to a master key system as none of the locks have a key that can open only that lock.
Skeleton keys have often been associated with attempts to defeat locks for illicit purposes, to release handcuffs for example, and standard keys have been filed down for that purpose. Legitimate skeleton or master keys are used in many modern contexts where lock operation is required and the original key has been lost or is not available. In hotels without electronic locks, skeleton keys are used by housekeeping services to enter the rooms.
Lever lock keys
In US English usage, 'skeleton key' is occasionally used to mean a standard lever lock key.This usage perhaps stems from such keys' resemblance to a skeletal figure, with the circle at top circumscribing the triangle at one-third the vertical length, resembling a stylized skull.
These keys were common in both cabinetry and door locks in early Colonial America and remained in common usage within the United States through the 1940s, giving way after World War II to the pin tumbler lock. Most keys of this type and their locks were formed of brass, although pewter was sometimes used. They were commonly available at hardware and other stores. Today, lever lock keys are associated with fine cabinetry.