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For the purposes of directing mail, the United Kingdom is divided by Royal Mail into postcode areas. The postcode area is the largest geographical unit used and forms the initial characters of the alphanumeric UK postcode. There are currently 121 geographic postcode areas in use in the UK and a further 3 often combined with these covering the Crown Dependencies of Guernsey, Jersey and Isle of Man.
Each postcode area is further divided into post towns and postcode districts. There are on average 20 postcode districts to a postcode area. The London post town is instead divided into several postcode areas.
The single or pair of letters chosen for postcode areas are generally intended as a mnemonic for the places served. Postcode areas, post towns and postcode districts do not follow political boundaries and usually serve much larger areas than the placenames with which they are associated. For example, within the PA postcode area the PA1 and PA78 postcode districts are 140 miles apart; and the eight postcode areas of the London post town cover only 40% of Greater London. The remainder of its area is covered by sections of twelve adjoining postcode areas: EN, IG, RM, DA, BR, TN, CR, SM, KT, TW, HA and UB.
|Postcode area||Postcode area name||Code formation|
|DG||Dumfries||Dumfries and Galloway|
|EC||East Central London|
|KT||Kingston upon Thames|
|ME||Rochester||Medway (now sometimes known as Maidstone)|
|NE||Newcastle upon Tyne|
|NW||North West London|
|SE||South East London|
|SW||South West London|
|WC||Western Central London|
|Postcode area||Postcode area name|
|IM||Isle of Man|
Glasgow, like London, was divided into compass districts: C, W, NW, N, E, SE, S, SW. When postcodes were introduced, these were mapped into the new G postcode: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on. As NW and SE had never been subdivided they became G20 and G40 respectively.
Dublin was split into Dublin postal districts by the Post Office in 1917 at the same time as other major cities in the UK like Liverpool and Manchester. After the creation of the Irish Free State (later the Republic of Ireland) the Irish government did not adopt postcodes; however, the Dublin postal districts remain to this day and the correct form of address is Dublin 7, etc. The postcode area D has not been re-used within the UK.
Norwich and Croydon were used for a postcode experiment in the late 1960s, which was replaced by the current system. The format was of the form NOR or CRO followed by two numbers and a letter, e.g. NOR 07A.
The BF postcode area was introduced in 2012 to provide optional postcodes for British Forces Post Office addresses, for consistency with the layout of other UK addresss. It uses the notional non-geographic post town "BFPO" and, as of 2012, the postcode district "BF1".
The non-geographic postcode area BX has been introduced for addresses which do not include a locality, this allows large organisations long-term flexibility as to where they receive their mail. This postcode area is used by Lloyds TSB (BX1 1LT) and the VAT Central Unit of HM Revenue and Customs (BX5 5AT).