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Master keying is a keying system in which more than one key can operate a lock or group of locks. Master keying is commonly used to restrict or allow access to a group of locks in a facility based on a user's access level. Various types of master keying exist depending on the type of lock and the number of access levels required. The use of master keying can have a dramatic effect on the number of real key differs and resistance to various forms of covert entry, such as lockpicking and impressioning.
Master Key Lock Designs
The type of lock will determine what master keying schemes are available. Each lock design has a different method of providing master keying either by adding components, modifying keys, or both.
- Traditional rotary combination locks can have have multiple true gates per wheel, allowing various combinations to be used. The majority of combination locks do not allow master keying without the addition of an auxillary override lock. Electronic combination locks can store a database of valid combinations for different users.
- Widened or additional true gates on the discs themselves can allow master keying. The high number of key differs in most disc-detainer locks minimizes the threat of key interchange in most systems.
- Various modifications to the levers themselves can allow master keying but most have an adverse effect on security. The majority of real-world lever locks that need to provide master keying do so by providing two keyholes with two sets of levers, or two keyholes with a single set of levers, each lever having two contact points.
- Additional, smaller master pins inserted between the top and bottom pins of each pin tumbler stack allows for additional potential shear lines. This so-called "split pin master keying" provides one of the most versatile master keying solutions. As more master pins are added, the chance of key interchange increases exponentially.
- Wafers are modified to have two contact points. The normal (change) key will pick up one set of contact points and master keys the other.
- Warded keys are modified to fit through different warding designs; the locks themselves are rarely modified.
Master Keying Schemes
- Construction keying
- A movable or breakable component is added to the traditional locking components. During "construction", users use keys that raise these components to the shear line. When the lock is ready to be used by the real owners, the components are removed or otherwise damaged so that the old keys no longer work. Construction keying is primarily used with pin-tumbler locks, though break-away components can be used with other designs, as well.
- Interchangeable Cores
- A lock that allows user-rekeying by physically removing the cylinder and replacing it with another. Interchangeable core locks usually feature two shear lines which operate independent of one another. One shear line is used to engage the bolt mechansism, and the other to operate the core removal mechanism.
- Maison Keying
- A master keying system where security is sacrificed to ensure a large number of users can easily access shared resources. This is usually performed by a removal of components or extreme master keying levels, both of which greatly reduce the security of the lock and keying systems. Maison keying is common in older schools and apartment buildings.
- Master Rings
- A secondary plug is located around the primary plug. The user keys operate the traditional shear line, but master keys operate the master ring shear line to open the lock. This is similar in function to interchangeable cores, but the master ring cannot be used to remove the cylinder.
- Partial Position Progression
- A method of master keying in which certain positions are progressed while other positions are held constant. With split-pin master keying, this means there are certain cuts on the keys which are the same on every key in the entire system and there are certain chambers in the locks which never have master pins in them. This method is very useful for small systems.
- Rotating Constant Progression System
- A method of master keying in which some positions are progressed and at least one position is held constant but the location of the constant is not fixed. This method is very useful for producing two-level systems with large numbers of change keys. Surprisingly, it produces more available change keys than total position progression does, in most cases. As with partial position progression, every key has at least one cut in common with the top master key and most locks have at least one chamber with no master pins in it.
- Sectional/Multiplex Keyways
- A master keying scheme where the keyway profile of a key is used to restrict physical entry of the key to one or more locks. Keys with low levels of access will have key profiles that cannot be inserted into higher level locks. Keys with high levels of access will have most of their warding removed, allowing them to be used in all or most locks in the system. This scheme is most commonly used in addition to traditional master keying schemes, but may be used alone. In warded locks, sectional keyways may be the only type of master keying available.
- Total Position Progression
- A method of master keying in which every position is progressed. With split-pin master keying, this means the lowest level change keys share no cuts in common with the top master key and most locks have a master pin in every chamber. This method is useful for producing systems with many levels of keying. Some consider it to be the default method to be used in every situation but it creates excessive incidental keys, which is contrary to ANSI standards.