Postcode

UK postal codes are known as postcodes. They are alphanumeric. The format of UK postcodes is generally:

LD DLL, LLD DLL, LDD DLL, or LLDD 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 45 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 British postal codes). BT is for Belfast but covers the whole of Northern Ireland.

Table of contents

1 London postcodes
2 Other cities' postcodes
3 Validation
4 Crown Dependencies
5 Overseas Territories
6 External links
7 See also

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 within the boundaries of Surrey County Council. And some of the postal districts cross county boundaries and even regional boundaries - the KT postcode covers areas in both Greater London and the South East Region.

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.

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.

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", which is generated by the Royal Mail and contains every UK address and postcode. It is commercially licensable and is often incorporated in address management packages.

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 -

Falkland Islands FIQQ 1ZZ
South Georgia and the South Sandwich Islands SIQQ 1ZZ
British Antarctic Territory BIQQ 1ZZ
St Helena STHL 1ZZ
Ascension Island ASCN 1ZZ

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.

External links

More detailed explanation of the postcode system
Explanation and lists of London postcodes
British postal codes - this PDF document from the Universal Postal Union explains the system and shows all allowable UK postcode formats.
History of the British postcode system (PDF format)