One of three types of locking mechanisms: Discrete: locks that are different from one another; each can only be opened by one key. Personal: a discrete lock, individually keyed, issued to an individual for his or her use in securing isolations. Series or system: a group of locks that are opened by a single key and are used in area/complex lockouts. Various sets of locks may be in use; however, only one key can open any given lock in a particular set