Master Keying: Hierarchical Access in Mechanical Key Systems

Levels of Keying

LIST defines each level by what the key operates:

• Change key. "A key which operates only one cylinder or one group of keyed alike cylinders in a keying system."
• Master key. "A key which operates all the master keyed locks or cylinders in a group, each lock or cylinder usually operated by its own change key."
• Grand master key. "The key which operates two or more separate groups of locks, which are each operated by a different master key."
• Great grand master key. The key that operates two or more groups, each run by a different grand master key.

The number of levels names the system. LIST calls a system with exactly three levels a grand master key system and one with exactly four a great grand master key system. Its table also gives key symbols for systems of five levels and more.

Standard key symbols

LIST's standard key coding system labels every key by its place in the hierarchy. In a three-level system:

Reading the symbol tells you the chain of authority. Key AA1 is a change key under master AA, which is under grand master A.

How One Cylinder Accepts Several Keys

A plain pin tumbler cylinder turns when the right key lifts each pin stack so that the gap between pins lines up with the shear line, where the plug meets the shell.

A master-keyed cylinder adds master pins to some chambers. LIST defines a master pin as a tumbler "placed between the top and bottom pin to create an additional shear line." Each pin stack can then be split at more than one height, so more than one key cut will let the plug turn. The change key splits the stacks at one set of heights; the master key splits them at another.

Every master pin creates a shear line on purpose, so pinning has to be planned. In a Locksmith Ledger column on keying errors, Rod Oden notes that a leftover master pin "develops an unexpected operating shear line." This page stays at the conceptual level. How pin sizes are calculated is a design task, and it is covered by the manufacturer's specifications and by the records described below. See key bitting.

Special Keys and Arrangements

Real systems often need exceptions to the simple tree. LIST names several:

• Selective master key. "An unassociated master key which can be made to operate any specific lock(s) in the entire system in addition to the regular master key(s) and/or change key(s) for the cylinder without creating key interchange."
• Maison key system. From the French for "house." A system "in which one or more cylinders are operated by every key (or relatively large numbers of different keys) in the system." LIST's example is the main entrance of an apartment building, which every suite key opens.
• Cross keying. "The deliberate process of combinating a cylinder (usually in a master key system) to two or more different keys which would not normally be expected to operate it together." Controlled cross keying stays under the same higher-level key. Uncontrolled cross keying spans different branches.
• Construction master key. "A key normally used by construction personnel for a temporary period during building construction. It may be rendered permanently inoperative without disassembling the cylinder." See construction keying.

Many master-keyed systems are built on interchangeable cores, which let a cylinder be swapped without taking the lock apart.

The Security Trade-Off

Master keying gives up some security for convenience, and the standard references say so. Because master pins add shear lines, a master-keyed cylinder opens with more than one key by design.

LIST's dictionary carries two warnings:

• Key interchange is "an undesirable condition, usually in a master key system, whereby a key unintentionally operates a cylinder or lock."
• Uncontrolled cross keying "severely limits the security of the cylinder and the maximum expansion of the system, and often leads to key interchange." LIST says controlled cross keying "could severely limit" both as well.

A lost high-level key also affects every lock beneath it in the hierarchy. That is why system owners pair master keying with key control: keys and records stay secret, and blanks are harder to get. Schlage's key-systems guide calls this "administrative security… barriers you put in place to prevent unwanted duplication." It lists its open Classic keyway as "duplicate anywhere," while its Everest 29 line "requires letter of authorization." Schlage says owners can apply "levels of administrative control on a door by door basis throughout a facility under a single keyway." See restricted keyways and key blanks and keyways.

Planning: The Keying Conference and ANSI/BHMA A156.28

LIST defines the keying conference as "a meeting of the end user and the keying system supplier at which the keying and levels of keying, including future expansion, are determined and specified."

In North America, the planning reference is ANSI/BHMA A156.28, Recommended Practices for Mechanical Keying Systems. The current edition is 2023. BHMA says it is written "for building owners, security professionals and others responsible for designing, implementing, and maintaining secure keying systems." It covers system design, gives guidelines for "the essential keying conference," sets good practices for key management, and aims to help owners "extend the life of keying systems to meet future demands."

The keying vocabulary also covers locks outside a master system. LIST defines keyed alike as two or more locks with the same combination and keyed different as locks each combinated differently. Either "may or may not be part of a keying system."

Records: The Schematic, Bitting List, and Key Records

A master key system depends on its paperwork. LIST names the core documents:

• Key system schematic. "A drawing with blocks utilizing keying symbols, usually illustrating the hierarchy of all keys within a master key system. It indicates the structure and total expansion of the system."
• Key bitting array. "A matrix (graphic) display of all possible bittings for change keys and master keys as related to the top master key."
• Bitting list. "A listing of all the key combinations used within a system."
• Key records. These typically include the bitting list, bitting array, schematic, end user, number of keys and cylinders issued, names of key holders, and the hardware or keying schedule.

Anyone holding these records could make keys for the system, so they are sensitive. In Europe, EN 1303:2026 (Cylinders and master key systems (MKS) for locks) points to a separate document, CEN/TS 17814, for "requirements and procedures to achieve and maintain protection of data and sensitive information related to mechanical Master Key Systems." See EN 1303.

Changing a System

Rekeying one cylinder inside a master system has to fit the existing hierarchy. Otherwise the new combination can cause key interchange with other doors. That is one reason A156.28 stresses planning for expansion at the keying conference. See rekeying locks and key cutting.

Related Topics

• Key control
• Construction keying
• Interchangeable cores
• Restricted keyways
• Key bitting
• Key blanks and keyways
• Pin tumbler locks
• Warded locks
• Rekeying locks
• EN 1303
• ANSI/BHMA lock grades

Sources