On Windows Server Systems, the domain controller (DC) is the server that responds to security authentication requests (logging in, checking permissions, etc.) within the Windows Server domain. In Windows NT, the central domain controller was known as the Primary Domain Controller (PDC), of which there could only be one with this role; all other domain controllers were identified as Backup Domain Controllers (BDC). In Windows 2000 and later the concept of primary and backup domain controllers was eliminated, partially to emphasize the multi-master replication technology available in Windows; however, there are still a number of master roles that only one domain controller can perform: schema master, domain naming master, infrastructure master, Relative ID master, and the PDC Emulator. These are called the Flexible single master operation roles. If one of these roles is lost the domain can still function, and if the server will not be available again the role may be seized. In Windows NT, a BDC can authenticate the users in a domain.
