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Postcodes in the United Kingdom

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UK postal codes are known as postcodes. They are alphanumeric. These codes introduced by the Post Office over a fifteen year period have been widely adopted by the UK public on the basis that as designed they assist with the delivery of mail.

However, as the format of the codes does not achieve its objective of primarily identifying the main sorting office and sub-office they have been surreptitiously replaced by a new system of five digit codes called Mailsort. Mail users who can deliver mail to the post office sorted by mailsort code receive discounts, whilst delivery by postcode provides no such incentive.

The format of UK postcodes is generally:

LD DLL
LLD DLL
LDD DLL
LLDD DLL
LLDL DLL
LDL DLL

where L signifies a letter and D a digit. It is a hierarchical system, working from left to right - the first letter or pair of letters represents the area, the following digit or digits represent the district within that area, and so on. Each postcode generally represents a street, part of a street, or a single premises. The part of the code before the space is the outward code used to direct mail from one sorting office to the destination sorting office (the alphabetic part identifying one of 121 postal districts), while the part after the space is the inward code used to sort the mail into individual postmen's delivery rounds, each separate code usually identifying the address to within 80 properties, although large businesses may have a unique code. The first letters of the postcode usually give some clue to its geographical location (but see London below), for example BS for Bristol and G for Glasgow (see list of postal codes in the United Kingdom). BT is for Belfast but covers the whole of Northern Ireland.

There is one exception (other than the overseas territories) to this rule; the postcode for the formerly Post Office-owned Girobank is GIR 0AA.

London postcodes

In the inner London area postcodes are slightly different, being based on the old system of London postal districts, which predated by many years the introduction of postcodes in the 1960s:

  • In central London, WC and EC (West Central and East Central)
  • In the outer part of inner London, N, NW, SW, SE, W and E.

Note that London postal districts rarely coincide with the boundaries of London boroughs (even the old, smaller boroughs). The numbering system also appears arbitrary on the map: for example, NW1 is close to central London, but NW2 is a long way out. This is because (after starting with 1 for the area closest to the centre) they were numbered alphabetically by the name of the district they represented.

In outer London the postcodes follow the more conventional pattern of postcodes deriving from the main sorting office. To confuse matters further many of these parts of London have traditionally had postal addresses that use the old county boundaries - for example postal addresses in Sutton traditionally read "Sutton, Surrey" and not "Sutton, London" even though Sutton is no longer a county borough in Surrey.

Several postcode areas cross county boundaries, covering parts of neighbouring counties as well as areas in Greater London. For example, KT (Kingston upon Thames), TW (Twickenham), and CR (Croydon) cover parts of Surrey, while DA (Dartford) and BR (Bromley) cover parts of Kent, and RM (Romford) covers part of Essex.

A further complication is that in some of the most central London areas, a further gradation has been necessary to produce enough postcodes, giving unusual codes like EC1A 1AA.

While most postcodes are allocated by administrative convenience, a few are deliberately chosen. For example in Westminster:

Other cities' postcodes

Until the 1960s, cities such as Belfast, Birmingham, Edinburgh, Glasgow, Leeds, Liverpool, Manchester, Newcastle upon Tyne and Sheffield were divided into different postal districts, each with a number, e.g. Toxteth in Liverpool was Liverpool 8. When the national postcode system was introduced, these were incorporated into the new postcodes, so that postcodes in that part of the city would start with L8, followed by the rest of the postcode. A similar system is still used in Dublin in the Republic of Ireland, the Dublin postal districts.

Validation

The consequence of the complexity outlined above is that for almost every rule concerning UK postcodes, an exception can be found which breaks that rule. Automatic validation of postcodes on the basis of pattern feasibility is therefore almost impossible to design, and the system contains no self-validating feature such as a check digit. Validation is usually performed against a copy of the "Postcode Address File" (PAF), which is generated by the Royal Mail and contains about 27 million UK commercial and residential addresses.

It is possible to validate the format of a postcode using the following rules:

  • The postcode must be of 6, 7, or 8 characters in length.
  • The postcode must include a space between the outward and inward codes.
  • The outward code (the set of characters to the left of the space) must be 2, 3, or 4 characters in length
  • The first character of the outward code must be alphabetic.
  • The inward code (the set of characters to the right of the space) must always be 3 characters in length.
  • The first character of the inward code must be numeric.
  • The second and third characters of the inward code must be alphabetic.

Application

The PAF is commercially licensable and is often incorporated in address management software packages. The capabilities of such packages allow an address to be constructed solely from the postcode and house number for most addresses. By including the map references of postcodes in the address database, the postcode can be used automatically to pinpoint a postcode area on a map. See http://www.streetmap.co.uk for an example of this in practice.

Crown Dependencies

The Channel Islands (Jersey and Guernsey) and the Isle of Man established their own separate postal administrations from the UK in 1969, and did not adopt postcodes until the early 1990s. Their postcodes follow the UK format, with Jersey being postcode area JE, Guernsey GY, and Isle of Man IM.

Overseas Territories

Some of the UK's overseas territories have their own postcodes -

Unlike UK postcodes, these are used for all addresses in those territories. The reason why they were introduced is because mail was often sent to the wrong place, e.g: St Helena to St Helens in England, Falklands to Falkirk in Scotland. In addition, many online companies would not accept addresses without a postcode. Mail from the UK continues to be treated as international, not inland, and sufficient postage must be used.


See also

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