Mark's Locksmith
Cruciform pin-tumbler locks may also use interchangeable cylinders, as do a few sophisticated lever locks.
Individually Keyed System (KD)
With an individually keyed system, each cylinder can be opened by its unique key.
Keyed Alike (KA)
This system allows for a number of cylinders to be operated by the same key. It is ideally suited to residential and commercial applications such as front and back doors.
Master Keyed (MK)
A master-keyed system involves each lock having its own individual key which will not operate any other lock in the system, but where all locks can be operated by a single master-key. This is usually applied in commercial environments.
Grand Master Keyed (GMK)
This is an extension of the master-keyed system where each lock has its own individual key and the locks are divided into 2 or more groups. Each lock group is operated by a master-key and the entire system is operated by one grand master-key. This is ideally utilized in complex commercial systems.
Common Entrance Suite / Maison Keying (CES)
This system is widely used in apartments, office blocks and hotels. Each apartment (for example) has its own individual key which will not open the doors to any other apartments, but will open common entrance doors and communal service areas. It is often combined with a Master Keyed system in which said key is kept by the landlord.