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Postcodes in the United Kingdom

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UK postal codes are known as postcodes.

UK postcodes are alphanumeric. These codes were introduced by the Royal Mail over a 15-year period from 1959 to 1974 — the full list is now available electronically from the Royal Mail as the Postcode Address File. They have been widely adopted not just for their original purpose of automating the sorting of mail but for many other purposes — see Postcode lottery.

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 supplemented by a newer 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.

Format

The format of UK postcodes is generally:

A9 9AA
A99 9AA
A9A 9AA
AA9 9AA
AA99 9AA
AA9A 9AA

where A signifies a letter and 9 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 or out code used to direct mail from one sorting office to the destination sorting office, while the part after the space is the inward code or in code used to sort the mail into individual delivery rounds, each separate code usually identifying the address to within 80 properties (with an average of 14 properties per postal code), although large businesses may have a unique code. The outward code can be split further into the area part (letters identifying one of 124 postal areas) and the district part (usually numbers); similarly, the inward code is split into the sector part (number) and the unit part (letters).

Name Location Component Format Number
postal area out code A or AA 124
postal district out code 9, 99 or 9A 3,064
sector in code 9 11,598
unit in code AA 1,780,000
Postcode Addresses 27,000,000

The letters in the outward code may give some clue to its geographical location (but see London below). For example, BS indicates Bristol and G indicates Glasgow; see List of postal areas in the United Kingdom for a full list. Although BT indicates Belfast, it covers the whole of Northern Ireland. The letters in the inward code, however, are restricted to the set ABDEFGHJLNPQRSTUWXYZ (excluding CIKMOV), which generally do not resemble digits or each other when hand-written.

There are at least two exceptions (other than the overseas territories) to this format:

London postcodes

Main article: London postal district

In the London area postcodes are slightly different, being based on the old system of 163 London postal districts and predating by many years the introduction of postcodes in the 1960s:

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

The London postal districts rarely coincide with the boundaries of the London boroughs (even the former, smaller metropolitan boroughs). The numbering system 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 containing the main sorting office) they were numbered alphabetically by the name of the main sorting office.

The area covered by the London postal districts was somewhat larger than the County of London, and included parts of Kent, Essex, Surrey, Middlesex and Hertfordshire. In 1965 the creation of Greater London caused this situation to be reversed as the boundaries of Greater London went far beyond the existing London postal districts.

Those places not covered by the existing districts received postcodes as part of the national coding plan, so the postcode areas of "EN" Enfield, "KT" Kingston upon Thames, "HA" Harrow, "UB" Uxbridge", "TW" Twickenham, "SM" Sutton, "CR" Croydon, "DA" Dartford, "BR" Bromley, "RM" Romford and "IG" Ilford cross administrative boundaries and cover parts of neighbouring counties as well as parts of London.

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

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

Postcode lottery

Postcodes were devised solely for the purposes of sorting and directing mail and rarely coincide with political boundaries. However, over time they have become a geographical reference in their own right with postcodes and postcode groups becoming synonymous with certain towns and districts. Further to this, the postcode has been used by organisations for other applications including government statistics, marketing, calculation of car and household insurance premiums and credit referencing.

There are several groups, mostly on the fringes of major population centres, who are affected in one way or another by the associations of their postcode. There is a movement in the Royal Borough of Windsor and Maidenhead to change the first two characters of their postcodes from "SL" to "WM" for vanity, so as not to be associated with Slough. A businessman in Ilford wishes to have the postcode of IG1 changed to E19 as he claims customers do not realise his business is based in London.

Residents of West Heath in SE2 wish to have their postcodes changed to that of adjacent Bexleyheath, citing higher insurance premiums as reason to change. Some residents of Kingston Vale in London SW15 wish to have their postcodes changed to adjacent Kingston upon Thames for the same reasons. Residents of Denham, Buckinghamshire have postcodes associated with Uxbridge, which causes confusion to delivery drivers.

In all these cases Royal Mail has said that there is "virtually no hope" of changing their postcode referring to their policy of changing postcodes only to match changes in their operations. [5] Under this policy residents of the Wirral Peninsula had their postcodes changed from the "L" (Liverpool) to "CH" (Chester) group when a new sorting office was opened. [6]

Some postcode areas straddle England's borders with Wales and Scotland. Examples of such postcodes include CH4, HR3, SY10, NP16 and TD15. This has led to British Sky Broadcasting subscribers receiving the wrong BBC and ITV regions, and newly-licensed radio amateurs being given incorrect callsigns.

Other cities' postcodes

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

A single numbering sequence was split between Manchester and Salford. Letters would be addressed to Manchester 1 or Salford 4. However in the 1960's, all the districts in both Manchester and Salford gained "M" postcodes, so "Salford 4" became M4, etc., much to the chagrin of Salfordians. The old coding lives on in a handful of street signs which are still embossed with "Salford 4" etc, at the bottom.

Glasgow was unique amongst UK cities as it had compass postal districts similar to London due to its status as the Second City of the British Empire, i.e., C, W, NW, N, E, S, SW, SE. When postcodes were introduced these were mapped into the new 'G' postcode area thusly: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on.

Validation

The consequence of the complexity outlined above is that for almost every rule concerning UK postcodes, an exception can be found. 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. However, even the PAF cannot be relied on as it contains errors, and because new postcodes are occasionally created and used before copies of the PAF can be distributed to users.

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

  • The postcode must be 6–8 characters in length (including the space)
  • The first character of the outward code must be alphabetic.
  • The outward code (the set of characters to the left of the space) must be 2–4 characters in length
  • The numeric portion of the outward code is not zero padded except when the number is zero.
  • The inward code (the set of characters to the right of the space) must always be 3 characters in length, the first numeric, the second and third alphabetic.

A regular expression that validates the format rather loosely is

^[A-Z][A-Z]?[0-9][A-Z0-9]? ?[0-9][ABDEFGHJLNPQRSTUWXYZ]{2}$

(this makes no assumptions about the space between the outward code and inward code)

All valid postcodes (except GIR 0AA and some of the Overseas Territories postcodes) will match this, but invalid postcodes will also match.

A somewhat more complete regular expression is:

^([A-PR-UWYZ][A-HK-Y0-9][AEHMNPRTVXY0-9]?[ABEHMNPRVWXY0-9]? {1,2}[0-9][ABD-HJLN-UW-Z]{2}|GIR 0AA)$

(this assumes that the space between the outward code and inward code is included, e.g. SW5 0QF)

However, this too is not completely accurate and will match many invalid postcodes. Yet more complete regular expressions are:

^(GIR 0AA)|([A-PR-UWYZ]\d|[A-HK-Y](?! )(\d|(?<=\d)[A-HJKSTUW])?((?<=[A-Z]{2}\d)[\dABEHMNPRV-Y])? \d[ABD-HJLNP-UW-Z]{2})$

(which will only work if lookarounds are supported by the regular expression engine being used)

^(GIR 0AA)|([A-PR-UWYZ]((\d(\d|[A-HJKSTUW])?)|([A-HK-Y]\d(\d|[ABEHMNPRV-Y])?)) \d[ABD-HJLNP-UW-Z]{2})$

(which does not use lookarounds but as a result has to be written in a non-linear fashion to keep track of the character position being tested)

None of these expressions come close to validating whether a postcode exists, as there are many otherwise valid "holes" that have not been assigned. Such a regular expression would be almost as long as the full list of postcodes. A regular expression matching just the existing outcodes is over a thousand characters long.

Application

The PAF is commercially licenseable 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.

Northern Ireland

Northern Ireland was the last part of the UK to be postcoded, between 1970 and 1974. While Belfast was already divided into postal districts, rural areas known as townlands posed an additional problem, as many roads were not named, and houses were not numbered. Consequently, many people living in such areas shared the same postal address, which still occurs in the Republic of Ireland.

Crown Dependencies

The Channel Islands (Jersey and Guernsey) and the Isle of Man established their own postal administrations separate 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.

British Forces Post Office (BFPO)

The British Forces Post Office (BFPO) is an agency that provides a postal service to HM Forces, separate from that provided by Royal Mail in the United Kingdom. BFPO addresses are used for the delivery of mail in the UK and around the world. BFPO codes such as BFPO 801 serve the same function as postal codes for civilian addresses.

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, Merseyside, and Ascension Island to Asunción, Paraguay. 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.

Postcode history

A Royal Mail Information Sheet (see External links below) describes how postcode trials were conducted in Norwich and Croydon.
(It is possible that these two towns were selected because they had the letter O in their names at positions where the designers were thinking of using a digit. For example they may have had in mind for Norwich the Canadian format of LDL DLD.) The Norwich scheme eventually used the initials NOR, for Norwich, and a further code related to each street and neighbourhood, with two numbers and a letter (e.g. NOR 52X, or NOR 88A). Norwich was subsequently completely re-coded but the scheme tested in Croydon was sufficiently close to the final design for it to be retained.

Another early adopter of postcodes was Newport. Here, Newport itself was allocated NPT, in a similar way to Norwich and Croydon, with the surrounding towns allocated NP1-NP8. This lasted into the mid 1980s when for operational reasons (NPT being non-standard, and too similar to NP7) it was recoded.

The legacy of the Croydon trial can still be seen today:

  • CR0 was the only postal district with a zero in that position: all others start with 1. This caused one of the PAF (see above) software products produced by the Royal Mail themselves to misbehave slightly! Subsequently, the "zeroth" district has been used in some other postcode areas, such as Bolton, Harrow and the Dengie peninsula in Essex.
  • A separate postal "district", CR9 is used for large users and PO Box holders. This policy has been used elsewhere, with normal postcodes "growing" upwards from district 1 and large-user postcodes "growing" downwards from district 99.
  • The CR0 district contains far more addresses than any other postal district in the country.
  • CR1 has never been used — possibly left spare for rationalisation. (The other CR districts, CR2 etc. were coded later and conform to the general standards.)
  • There was at one point a movement to change all CR0 postcodes to CR1, but this was rejected.
  • CR0 is often incorrectly written as CRO, although in some type faces the digit '0' and letter 'O' are identical -- the problem is exacerbated as it is often pronounced 'Sea Arr Oh' rather than 'Sea Arr Zero'.

See also

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