A master key operates a set of several locks. Usually, there is nothing special about the key itself, but rather the locks into which it will fit. These master-keyed locks are configured to operate with two, or more, different keys: one specific to each lock (the change key), which cannot operate any of the others in the set, and the master key, which operates all the locks in the set. Locks that have master keys have a second set of the mechanism used to operate them that is identical to all of the others in the set of locks. For example, master keyed pin tumbler locks often have two shear points at each pin position, one for the change key and one for the master key. A far more secure (and more expensive) system has two cylinders in each lock, one for the change key and one for the master key.

Master keyed lock systems generally reduce overall security. The fact that some pin chambers have two shear points allows for more options when picking and it also allows for more keys to operate. For example, a standard 6 pin cylinder, which was designed to be operated by only one key, can be operated by up to 2⁶ = 64 keys if there are two shear points in each chamber.

Larger organizations, with more complex systems, may have several levels of master keys, where the top level key works in all of the locks in the system. To visualize this, it can be thought of as a hierarchical chart, or a tree.

A practical attack exists to create a working master key for an entire system given only access to a single master-keyed lock, its associated change key, a supply of appropriate key blanks, and the ability to cut new keys.

Locksmiths may also determine cuts for a replacement master key, when given several different key examples from a given system.