Non-geographic postcode

Postal codes used in the United Kingdom are known as postcodes (originally postal codes). They are alphanumeric and were adopted nationally between 11 October 1959 and 1974, having been devised by the GPO (Royal Mail). A full postcode is known as a "postcode unit" and designates an area with a number of addresses or a single major delivery point.

Postcodes have been adopted for a wide range of purposes in addition to aiding the sorting of the mail: for calculating insurance premiums, designating destinations in route planning software and as the lowest level of aggregation in census enumeration. The boundaries of each postcode unit and within these the full address data of currently about 29 million addresses (delivery points) are stored, maintained and periodically updated in the Postcode Address File database.

The initial system of named postal districts, developed in London and other large cities from 1857, evolved towards the present form: in 1917 London was split into broad numbered subdivisions, and this extended to the other cities in 1934.

The structure of a postcode is a one or two-letter postcode area code named for a local town or area of London, one or two digits signifying a district in that region, a space, and then an arbitrary code of one number and two letters. For example, the postcode of the University of Roehampton in London is SW15 5PU, where SW stands for south-west London. The postcode of GCHQ is GL51 0EX, where GL signifies the postal town of Gloucester. The postal town refers to a wide area and does not relate to a specific town, county or region. GL51 is one of the postcodes for the town of Cheltenham (in Gloucestershire) which is where GCHQ is located.

The London post town covers 40% of Greater London. On inception (in 1857/8) it was divided into ten postal districts: EC (East Central), WC (West Central), N, NE, E, SE, S, SW, W, and NW. The S and NE sectors were later abolished. In 1917, as a wartime measure to improve efficiency, these were subdivided with a number applied to each sub-district, with the area served directly by the district head office as "1" with the others allocated alphabetically by delivery office, leading to N2 East Finchley delivery office, N3 Finchley delivery office, N4 Finsbury Park delivery office. These divisions changed little, usually only changed for operational efficiency.

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