Postcode

LN NLL, LLN NLL, LNN NLL, or LLNN NLL

where L signifies a letter and N 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 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.

London postcodes

London 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 outer 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. Also, there are places within Greater London that don't have "London" postcodes (eg, Enfield) - because of the expansion of London over time. 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.

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.

Overseas dependencies 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

External links

• More detailed explanation of the postcode system (http://www.evoxfacilities.co.uk/evoxps.htm)
• Explanation and lists of London postcodes (http://www.brainstorm.co.uk/public/utils/PostLondon)
• British postal codes (http://www.upu.int/post_code/en/GBR.pdf) - this PDF document from the Universal Postal Union explains the system and shows all allowable UK postcode formats.
• History of the British postcode system (http://www.consignia.com/heritage/downloads/infosheet_4.pdf) (PDF format)