text: When the national postcode system was introduced, many existing postal districts were incorporated into it, so that postcodes in Toxteth (Liverpool 8) start with L8. The districts in both Manchester and Salford gained M postcodes, so Salford 7 became M7 and so on (and similarly in Brighton and Hove, both using the prefix BN). The old coding lives on in a small number of street signs with (for example) "Salford 7" at the bottom. In other cases, the district numbers were replaced with unrelated numbers. In Glasgow many of its G-prefixed numbers are not used as C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on. In London (as postally defined), 1917-created postal districts are mapped unchanged to those of today but its much enlarged administrative area, Greater London, was created in April 1965. From that month the remaining 60% of Greater London's area has postcodes referring to 13 other post towns. Additionally, there were too few postcodes to adequately cover districts in central London (particularly in the WC and EC areas), so these were subdivided with a letter suffix rather than being split into new numbered districts so as to retain the familiar codes.
  GB postcodes available as OpenData
  Prior to 1 April 2010, the Royal Mail licensed use of the postcode database for a charge of about £4,000 per year. Following a campaign and a government consultation in 2009, the Ordnance Survey released Code-Point Open, detailing each current postcode in Great Britain together with a geo-code for re-use free of charge under an attribution-only licence (Open Government Licence as part of OS OpenData).
  Postcodes linked to a variety of UK geographies
  The Office for National Statistics (ONS Geography) maintains and publishes a series of freely available, downloadable postcode products that link all current and terminated UK postcodes to a range of administrative, health, statistical and other geographies using the Code-Point Open grid reference.
  Formatting
  Overview
  The postcodes are alphanumeric, and are variable in length: ranging from six to eight characters (including a space). Each postcode is divided into two parts separated by a single space: the outward code and the inward code respectively. The outward code includes the postcode area and the postcode district, respectively. The inward code includes the postcode sector and the postcode unit respectively. Examples of postcodes are "SW1W 0NY", "PO16 7GZ", "GU16 7HF", and "L1 8JQ".