A telephone exchange name or central office name was a distinguishing and memorable name assigned to a central office. It identified the switching system to which a telephone was connected. Each central office served a maximum of 10,000 subscriber lines identified by the last four digits of the telephone number. Areas or cities with more subscribers were served by multiple central offices, possibly hosted in the same building. The leading letters of a central office name were used as the leading components of the telephone number representation, so that each telephone number in an area was unique. These letters were mapped to digits, which was indicated visibly on a dial telephone.