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. 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, unlocking or locking the lock. To counteract the illicit creation of such keys lockmakers can place wards not just in the centre but on the outside as well, making attempts at creating a successful skeleton key harder.
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 will have all the levers heights the same but each door will have different wards. A skeleton key has the warded section of the key removed so that it opens all the doors of a system. It is not to be confused with a system where all the locks are keyed alike (one type of key will open all locks but each lock does not have its own unique key that will open only it).
Skeleton keys have often been associated with attempts to defeat locks for illicit purposes, for example to release handcuffs, and standard keys can be filed down for that purpose. Legitimate skeleton or master keys are widely used in many modern contexts where unlocking is required even when the original key has been lost or is not available. In hotels, such keys can be used to allow housekeeping services to enter a room.
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.