Traditionally, central office switching systems were designed to serve up to ten thousand subscriber numbers. Thus, subscribers were assigned four-digit line or station numbers. This rounded out the total number of digits in a subscriber telephone number to ten: a three-digit area code, three-digit central office code, and four digits for each line. This defined the North American Numbering Plan as a "closed numbering plan", as opposed to developments in some other countries where the number of digits was not fixed.