A master key system is a locking system composed of not just one, but multiple different locking cylinders. Each individual cylinder has a functional and practical relationship to the others.

A distinction is made between:

Keyed-alike systems (multiple locks of the same type that can be opened with a single key)
Central lock systems (abbreviated as Z-systems; they usually consist of several central locks that can be opened by all keys within the system)
Master key systems (abbreviated HS systems; they operate with one or more superordinate keys that can open all cylinders within the system)
Central master key systems (abbreviated Z/HS systems; they combine the features of both central lock and master key systems)
Grand master key systems (abbreviated GHS systems; expanded master key systems with additional hierarchy levels)

Central lock systems are commonly used in apartment buildings. Master key systems and grand master key systems are often found in larger company buildings.

Locking systems offer the advantage that there can be a master key, which authorized persons must handle with particular care.

Electronic systems generally include special access authorizations. For example, access can be granted temporarily, or individual keys can be blocked for certain time periods or permanently—without the need to replace any cylinder.