A Linked Numbering Scheme (LNS) is a telephone numbering plan applied to an area of the country where calling between lines on a number of adjacent exchanges is done without using a dialing code.
The largest LNS in the UK is that for the London telephone area - formerly known as the London Director area. Here, several million subscribers can call each other by dialing a uniform code. Thus, for example, anyone calling from an (020) number can reach Transport for London travel enquiries by dialing 7222 1234.
Smaller schemes apply outside London. Uxbridge, for example, has the subscriber trunk dialing (STD) code 01895. Uxbridge exchange is the parent for Denham, Harefield, Ruislip and West Drayton; anyone connected to any of those exchanges can call any of the others without having to prefix the number with 01895. This is achieved by giving subscriber lines on each exchange different prefix numbers, thus: all numbers are six-figure; Denham numbers start with 83, Harefield with 82, Ruislip with 6 and West Drayton with 4. Uxbridge numbers start with 2 or 81. All calls must have all six digits dialled - even if a subscriber is on Denham exchange and is calling another subscriber on Denham exchange, they must still dial 83xxxx.
Incoming calls from any other exchange for a subscriber on any of the five exchanges must all be prefixed with the same 01895 code.
In the United States, from the creation of the Washington, D.C. area code 202 in 1947 until 1991 when the number of telephone lines exceeded the capacity of one area code, all persons in the City of Washington as well as the adjoining communities in the U.S. States of Maryland and Virginia, could call any number in the region by simply dialing the 7 digit local number, even if the number was in a different state. The way this was done was, in the adjoining counties in Maryland and Virginia, in addition to the area code 703 or area code 301 numbers, they would also be assigned the same local number in the 202 area code. This meant a 585-xxxx number in Silver Spring, Maryland would be reachable from outside the area either as a 202-585-xxxx or 301-585-xxxx, but would be called from DC or Virginia as 585-xxxx. It also meant that any prefix (such as 585 in the above example) in the entire region could not be duplicated.
In some other rural areas in the U.S. where local calling areas cross state lines, numbers in adjoining areas could be called by dialing the 7-digit number without the area code. If the same prefix existed in the caller's area code, they would have to dial it as a long-distance number.