A Master Key is created when a group of locks are "custom" coded so each lock uses a different key, but a combination is "added" so one key will open all locks. It is a myth that there exists a particular Master Key that will open all locks. In order to Master-key a group of locks the locks must all be made by the same manufacturer or at least have "common" Keyways. In a group of locks that are Master-keyed (MK) each lock is operable by both it's own individual key and the Master-Key. Each individual key operates it's own lock only. The Master-Key operates all the locks in the group. Other systems are also possible using sub-Masters, Grand-Masters, etc.