Postcodes in the United Kingdom

A full postcode is known as a "postcode unit" and designates an area with several addresses or a single major delivery point.^[1] The structure of a postcode is two alphanumeric codes that show, first, the post town and, second, a small group of addresses in that post town. The first alphanumeric code (the outward code or outcode) has between two and four characters and the second (the inward code or incode) always has three characters. The outcode indicates the postcode area and postcode district. It consists of one or two letters, followed by one digit, two digits, or one digit and one letter. This is followed by a space and then the incode which indicates the postcode sector and delivery point (usually a group of around 10-15 addresses,^{[citation needed]} but can vary). The incode (always three characters), starts with a number (denoting a sector within the district), and ends with two letters (denoting delivery points which are allocated to streets, sides of a street or individual properties).

Postcode areas are usually, but not always, named after a major town or city – such as B for Birmingham. Some are named after a smaller town (e.g. Southall postcode area is UB after Uxbridge) or a combination of towns (e.g. SM appears to be named after Sutton and Morden). A small number are regional – such as HS for the Outer Hebrides, FY for Fylde (the region around Blackpool) and ME for the Medway conurbation, with Rochester as its main post town. In the case of London (a Post Town), there is not a single "London" postcode area (such as "LO"); rather there are eight (N, E, EC, SE, SW, W, WC and NW) reflecting the preceding system for coding London based on compass points. In the case of Northern Ireland, the entire province has a single postcode area BT (named for Belfast). The mnemonic features various combinations - most commonly, first two letters (CH for CHester); first and last letters (BH for BournemoutH); first and key syllable letters (IV for InVerness). Postcodes generally do not align with historical county or local authority boundaries, and can also cross national boundaries (e.g. the CH and TD postcode areas).

Each postcode area contains a number of post towns and postcode districts. All districts are defined by either one or two digits (AA9 or AA99); in London (only), some districts are additionally defined by one digit and one letter (A9A or AA9A).

As a general rule, postcode districts are numbered consecutively from 1, with the exceptions of:

• numbering from 0: BL, BS, CM, CR, FY, HA, PR, SL and SS
• numbering from 10: AB, LL, SO

In most postcode areas, the central part of the post town for which the postcode area is named will have the district number 1 e.g. B1 (central Birmingham). In addition, the postcode "aa1 1AA" was often allocated to the crown or principal post office in the central postcode district.

The allocation of postcode district numbers, in most cases, is from the main post town outward e.g. Halifax which has seven districts numbered consecutively HX1 to HX7, but this is not universal. There are a number of different numbering allocations and exceptions:

• geographical direction: e.g. the Outer Hebrides postcode area HS, where the districts are numbered from north to south
• geographic reality: particularly in larger postcode areas, the scale of the districts discourages consecutive numbering e.g. in the NE postcode area, NE50 to NE60 are not allocated due to the substantial distance between NE49 (Haltwhistle) and the next district to be numbered (Morpeth, which was numbered NE61 rather than NE50), recognising the geographic separation of districts in the west (up to NE49) from those to the north (NE61 upwards).
• clusters: particularly if the postcode area encompasses several major towns or cities (e.g. S postcode area with Sheffield S1–S36, Chesterfield S40–S49, Rotherham S60–S65, Barnsley S70–S75 and Worksop S80–S81)
• alphabetical order of district sub-offices: used only in London postcode areas (e.g. E2 is Bethnal Green, E3 is Bow, E4 is Chingford etc., which results in intuitive anomalies such as SE1 and SE2 being far from neighbours, with SE1 being large part of Central London south of the Thames (Waterloo and the borough of Southwark) and SE2 covering Abbey Wood at the far eastern end of the Elizabeth Line (in the boroughs of Greenwich and Bexley).
• earlier districts: where preceding postal districts have been adopted into the current system (such as Glasgow's "compass points") or translated from the postcode pilot phase (such as Croydon's CRO becoming CR0 rather than CR1).

• new postcode districts: new postcode districts are occasionally created, usually due to increased demand for addresses following housing/business development and exhaustion of available postcodes for the existing district. There appear to be no clear rules for numbering new districts. If no sequential numbers are available, then new non-consecutive numbers are allocated. For example, in the CV postcode area, the CV47 district was formed in 1999 from parts of the CV23 and CV33 districts, where the highest numbered district then allocated was CV37.
• transfer of postcode districts: postcode districts can be transferred for operational reasons e.g. PH49 and PH50 postcode districts which resulted from the transfer and recoding of PA39 and PA40 districts (PH39 and PH40 were already allocated although PH45–PH48 were not allocated). This also occurred with the creation of the HS postcode area, the only new geographic postcode area created since 1973, from the previous PA80 to PA87 districts.

London uniquely uses letters for subdivisions of some of its postcode districts (E1, N1, W1, WC1, WC2, EC1–EC4, NW1, SE1, SW1) with letters used substantively in three areas: W (W1), WC (WC1 and WC2) and EC (EC1–EC4), these being the most central areas with the densest concentration of addresses. These are effectively postcode districts in their own right, and could have been given double digit numbers (e.g. for WC in the range WC10-WC19 and WC20-WC29). The districts are geographically extremely small. Using double-digit numbering might have encountered resistance due to their "micro-size" and also due to the long-standing use of "WC1" etc. which signified in some cases a cultural or wealth status which may have been diluted or lost if re-coded from WC1 to, say, WC15. Even if London had dropped the initial compass points systems to create a single post code area, the challenge of the large number of districts (169) would have required a bespoke numbering system:

Accordingly, many postcode districts are not physically contiguous, despite the inference from their numbering. Likewise, the centrality of a postcode district within a postcode area cannot be reliably inferred from the postcode alone. See postcode area.

Postcodes have been adopted for a wide range of purposes in addition to aiding the sorting of mail: for calculating insurance premiums, designating destinations in route planning software and as the lowest level of aggregation in census enumeration. The boundaries of each postcode unit and within these the full address data of currently about 29 million addresses (delivery points) are stored, maintained and periodically updated in the Postcode Address File database.^[1]

Theoretically, deliveries can reach their destination using the house number (or name if the house has no number) and postcode alone; however, this is against Royal Mail guidelines, which request the use of a full address.^[3]

The London post town covers 40% of Greater London. On inception (in 1857/8), it was divided into ten postal districts: EC (East Central), WC (West Central), N, NE, E, SE, S, SW, W, and NW. The S and NE sectors were later abolished. In 1917, as a wartime measure to improve efficiency, each postal district was subdivided into sub-districts each identified by a number; the area served directly by the district head office was allocated the number 1; the other numbers were allocated alphabetically by delivery office, e.g. N2 East Finchley delivery office, N3 Finchley delivery office, N4 Finsbury Park delivery office etc. Since then these sub-districts have changed little.

Some older road signs in Hackney still show the North East (NE) sector/district.

Following the successful introduction of postal districts in London, the system was extended to other large towns and cities. Liverpool was divided into Eastern, Northern, Southern and Western districts in 1864/65, and Manchester and Salford into eight numbered districts in 1867/68.^[4]

In 1917, Dublin—then still part of the United Kingdom—was divided into numbered postal districts. These continue in use in a modified form by An Post, the postal service of the Republic of Ireland. In 1923, Glasgow was divided in a similar way to London, with numbered districts preceded by a letter denoting the compass point (C, W, NW, N, E, S, SW, SE).^[4]

In January 1932 the Postmaster General approved the designation of some predominantly urban areas into numbered districts.^[4] In November 1934 the Post Office announced the introduction of numbered districts (short postal codes) in "every provincial town in the United Kingdom large enough to justify it". Pamphlets were issued to each householder and business in ten areas notifying them of the number of the district in which their premises lay. The pamphlets included a map of the districts, and copies were made available at local head post offices. The public were "particularly invited" to include the district number in the address at the head of letters.^[5]

A publicity campaign in the following year encouraged the use of the district numbers. The slogan for the campaign was "For speed and certainty always use a postal district number on your letters and notepaper". A poster was fixed to every pillar box in the affected areas bearing the number of the district and appealing for the public's co-operation. Every post office in the numbered district was also to display this information. Printers of Christmas cards and stationery were requested to always include district numbers in addresses, and election agents for candidates in the upcoming general election were asked to ensure they correctly addressed the 100 million items of mail they were expected to post. Businesses were issued with a free booklet containing maps and listings of the correct district number for every street in the ten areas.^[6]

The ten areas were:^[6]

• Birmingham
• Brighton/Hove
• Bristol
• Edinburgh
• Glasgow
• Leeds/Bradford
• Liverpool
• Manchester/Salford
• Newcastle upon Tyne
• Sheffield

For example, Toxteth was Liverpool 8. A single numbering sequence was shared by Manchester and Salford: letters would be addressed to Manchester 1 or Salford 7 (lowest digits, respectively). Some Birmingham codes were sub-divided with a letter, such as Great Barr, Birmingham 22 or Birmingham 22a,^[7] as can still be seen on many older street-name signs.

The Post Office experimented with electromechanical sorting machines in the late 1950s.^[8] These devices presented an envelope to an operator, who would press a button indicating which bin to sort the letter into. Postcodes were suggested to increase the efficiency of this process by removing the need for the sorter to remember the correct sorting for as many places.^[9] In January 1959 the Post Office analysed the results of a survey on public attitudes towards the use of postal codes, choosing a town in which to experiment with codes. The envisaged format was a six-character alphanumeric code with three letters designating the geographical area and three numbers to identify the individual address.^[10] On 28 July Ernest Marples, the Postmaster General, announced that Norwich had been selected, and that each of the 150,000 private and business addresses would receive a code by October. Norwich had been selected as it already had eight automatic mail sorting machines in use.^[11] The original Norwich format consisted of "NOR", followed by a space, then a two-digit number (which, unlike the current format, could include a leading zero), and finally a single letter (instead of the two final letters in the current format).^[12]

In October 1965, Tony Benn as Postmaster General^[13] announced that postal coding was to be extended to the rest of the country in the next few years.^[14]

On 1 May 1967 postcodes were introduced in Croydon. The many postcodes for central Croydon began with "CRO", while those of the surrounding post towns with CR2, CR3 and CR4. The uniform system of a set of three final characters after the space (such as 0AA, known as the inward code) was adopted. This was to be the beginning of a ten-year plan, costing an estimated £24 million. Within two years it was expected that full coding would be used in Aberdeen, Belfast, Brighton, Bristol, Bromley, Cardiff, Coventry, Manchester, Newcastle upon Tyne, Newport, Reading, Sheffield, Southampton and the Western district of London.^[15] By 1967, codes had been introduced to Aberdeen, Southampton, Brighton and Derby.^[16] In 1970, codes were introduced to the London Western and North Western postal districts.^[17] In December 1970, much Christmas mail was postmarked with the message "Remember to use the Postal Code" although codes were used to sort mail in only a handful of sorting offices.^[18]

During 1971, occupants of addresses began to receive notification of their postcode. Asked in the House of Commons about the completion of the coding exercise, the Minister of State for Posts and Telecommunications (whose role superseded that of Postmaster General in 1969), Sir John Eden, stated that it was expected to be completed during 1972.^[19] The scheme was finalised in 1974 when Norwich was completely re-coded but the scheme tested in Croydon was sufficiently close to the final design for it to be retained, with CRO standardised as CR0 (district zero) thus removing the need to create a CR1 district.^[4]

A quirk remained: the central Newport (Gwent) area was allocated NPT at a similar time to Croydon becoming CRO, and surrounding areas were (as today) allocated NP1–NP8. NPT lasted until the end of 1984 when it was recoded NP9.^[20]

Girobank's GIR 0AA was the last domestic postcode with a fully alphabetical outward code. That code no longer exists in the Royal Mail's PAF system, but was taken over by the bank's current owners, Santander UK.^[21]

When the national postcode system was introduced, many existing postal districts were incorporated into it, so that postcodes in Toxteth (Liverpool 8) start with L8. The districts in both Manchester and Salford gained M postcodes, so Salford 7 became M7 and so on (and similarly in Brighton and Hove, both using the prefix BN). The old coding lives on in a small number of street signs with (for example) "Salford 7" at the bottom. In other cases, the district numbers were replaced with unrelated numbers. In Glasgow many of its G-prefixed numbers are not used following the transposition of the earlier compass point districts to "G" districts: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on. In London (as postally defined), 1917-created postal districts are mapped unchanged today despite Greater London, created in April 1965, covering a much larger administrative area. The London post town covers 40% of Greater London and the remaining 60% of Greater London's area has postcodes referring to 13 other post towns. Additionally, there were too few postcodes to adequately cover districts in central London (particularly in the WC and EC areas), so these were subdivided with a letter suffix rather than being split into new numbered districts so as to retain the familiar codes.

Prior to 1 April 2010, the Royal Mail licensed use of the postcode database for a charge of about £4,000 per year.^[22] Following a campaign and a government consultation in 2009,^[23] the Ordnance Survey released Code-Point Open, detailing each current postcode in Great Britain together with a geo-code for re-use free of charge under an attribution-only licence (Open Government Licence as part of OS OpenData).

The Office for National Statistics (ONS Geography) maintains and publishes a series of freely available, downloadable postcode products that link all current and terminated UK postcodes to a range of administrative, health, statistical and other geographies using the Code-Point Open grid reference.

The postcodes are alphanumeric, and are (possibly uniquely) variable in length: ranging from six to eight characters (including a space). Each postcode is divided into two parts separated by a single space: the outward code and the inward code respectively. The outward code includes the postcode area and the postcode district, respectively. The inward code includes the postcode sector and the postcode unit respectively. Examples of postcodes are "SW1W 0NY", "PO16 7GZ", "GU16 7HF", and "L1 8JQ".^[24]

The outward code is the part of the postcode before the single space in the middle. It is between two and four characters long. Examples of outward codes are "L1", "W1A", "RH1", "RH10" or "SE1P". A few outward codes are non-geographic, not divulging where mail is to be sent. These postcodes cannot be used for navigation purposes.

The postcode area is part of the outward code. The postcode area is either one or two characters long and is alphabetical, with there being 121 such areas. Examples of postcode areas are "L" for Liverpool, "RH" for Redhill and "EH" for Edinburgh. A postal area may cover a wide area, for example "RH" covers various settlements in eastern Surrey and north eastern West Sussex, and "BT" (Belfast) covers the whole of Northern Ireland.

The postcode district is one digit, two digits or a digit followed by a letter.

The inward code is the part of the postcode after the single space in the middle. It is three characters long. The inward code assists in the delivery of post within a postal district. Examples of inward codes are "0NY", "7GZ", "7HF", or "8JQ".^[25]

The postcode sector is made up of a single digit (the first character of the inward code). Most postcode areas do not use all of the sectors 0–9 in order to allow for the possibility of more sectors being added in the face of new development. Rather, in the initial allocation of postcodes, neighbouring postcode districts were often assigned to contain the ten sectors between them. For example, across the three postcode districts BS6-8 (which are next to each other in north west Bristol), sectors 1-4 were assigned to BS8, sectors 5-7 were assigned to BS6, and sectors 8-9 and 0 were assigned to BS7 (more recent changes have resulted in all three of those areas now having a sector 9).^[26]

The postcode unit is two characters added to the end of the postcode sector. A postcode unit generally represents a street, part of a street, a single address, a group of properties, a single property, a sub-section of the property, an individual organisation or (for instance Driver and Vehicle Licensing Agency) a subsection of the organisation. The level of discrimination is often based on the amount of mail received by the premises or business.

The format is as follows, where A signifies a letter and 9 a digit:

Notes:

• As all formats end with 9AA, the first part of a postcode can easily be extracted by ignoring the last three characters.
• Areas with only single-digit districts: BL, BR, FY, HA, HD, HG, HR, HS, HX, JE, LD, SM, SR, WC, WN, ZE (although WC is always subdivided by a further letter, e.g. WC1A)
• Areas with only double-digit districts: AB, LL, SO (for AB this arose from decoding of the original five districts AB1-AB5 by adding a second digit, to enable additional postcodes to become available, thus AB1 was divided into AB10-AB16).
• Areas with a district '0' (zero): BL, BS, CM, CR, FY, HA, PR, SL, SS (BS is the only area to have both a district 0 and a district 10)
• The following central London single-digit districts have been further divided by inserting a letter after the digit and before the space: EC1–EC4 (but not EC50), SW1, W1, WC1, WC2 and parts of E1 (E1W), N1 (N1C and N1P), NW1 (NW1W) and SE1 (SE1P).
• The letters Q, V and X are not used in the first position.
• The letters I, J and Z are not used in the second position.
• The only letters to appear in the third position are A, B, C, D, E, F, G, H, J, K, P, S, T, U and W when the structure starts with A9A.
• The only letters to appear in the fourth position are A, B, E, H, M, N, P, R, V, W, X and Y when the structure starts with AA9A.
• The final two letters do not use C, I, K, M, O or V, so as not to resemble digits or each other when hand-written.
• Postcode districts are one of ten digits: 0 to 9, with 0 only used once 9 has been used in a post town, save for Croydon (see above).
• Postcode sectors can also be one of ten digits: 0 to 9, though in some postcode areas the 0 is the beginning of the sequence (for example in LE), while in other areas it is the end of the sequence (i.e. 10, as in CV).

A postcode can be validated against a table of all 1.7 million postcodes in Code-Point Open. The full delivery address including postcode can be validated against the Royal Mail Postcode Address File (PAF), which lists 29 million valid delivery addresses,^[27] constituting most (but not all) addresses in the UK.^[28] A regular expression for validating UK postcodes is specified in the British Standards document BS 7666.^[29]

All or part of one or more postcode districts are grouped into post towns.^[30] Larger post towns may use more than one postcode district, for example Crawley uses RH10 and RH11. In a minority of cases, a single number can cover two or more post towns – for example, the WN8 district includes Wigan and Skelmersdale post towns; and the GL17 district contains five post towns.

The Channel Islands and the Isle of Man established their own postal administrations separate from the UK in 1969. Despite this, they adopted the UK-format postcodes in 1993–94: Guernsey using GY, the Isle of Man using IM, and Jersey using JE.^[31]

The independent jurisdiction of Sark was assigned a unique postcode district GY10 in 2011 to differentiate it from Alderney. The CEO of Guernsey Post, Boley Smillie, said "this has been a long time coming" and "... Sark should have had its own identity back then [when postcodes were adopted in 1993]".^[32]

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Some of the British Overseas Territories have postcodes that broadly follow the format of the UK postcode system or, in Gibraltar's case, adopts the UK format (and effectively creates a new postcode area GX). The four-letter outward codes conflict with the two letter outward codes in some respects - the use of I as the second letter (BIQQ, FIQQ, SIQQ), and duplication with existing postcode areas (BBND and BB Blackburn, STHL and ST Stoke-on-Trent, TDCU and TD Tweeddale):

These were introduced because mail was often sent to the wrong place, e.g., to St Helena instead of St Helens, Merseyside^[39] or St Helens, Isle of Wight.^[40] and to Edinburgh instead of Edinburgh, Tristan da Cunha, and many online companies would not accept addresses without a postcode.^[37] Mail from the UK continues to be treated as international, not inland, and sufficient postage must be used.^[41]

Bermuda has developed its own postcode system, with unique postcodes for street and PO Box addresses,^[42] as have the Cayman Islands,^[43] Montserrat and the British Virgin Islands.^[44] Montserrat recently introduced postal codes,^[45] and a system has been under consideration in Gibraltar^[46] with the code GX11 1AA being introduced as the generic postcode for the territory in the interim.^[36][47]

The separate postal code systems for those territories are shown below:

The British Sovereign Base Areas of Akrotiri and Dhekelia in Cyprus use Cypriot postal codes for civilian use. The British military use BFPO addresses.

The British Forces Post Office (BFPO) provides a postal service to HM Forces separate from that provided by Royal Mail in the United Kingdom, with BFPO addresses 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, with the last line of the address consisting of "BFPO" followed a space and a number of 1 to 4 digits.

For consistency with the format of other UK addresses, in 2012 BFPO and Royal Mail jointly introduced an optional alternative postcode format for BFPO addresses, using the new non-geographic postcode area "BF" and the notional post town "BFPO". Each BFPO number is assigned to a postcode in the standard UK format, beginning "BF1". Inward codes are assigned: 0 – Germany, 1 – UK, 2 – Rest of Europe, 3 – Rest of World, 4 – Ships and Naval Parties, 5 – Rest of World, Operations and Exercises, 6 – Rest of World, Operations and Exercises.^[49] The database was released commercially in March 2012 as part of the Royal Mail Postal Address File (PAF).^[50][51] A postcode is not required if the traditional "BFPO nnnn" format is used.

Overview: Almost all postcodes areas and almost all postcodes apply to a geographic area (buildings or streets) but some (areas, districts and specific postcodes) are used only for sorting/routing and thus cannot be used for identifying location, estimating distance or route finding in SatNav systems.^[52] They are commonly termed “non-geographic” postcodes in the sense that they refer not to physical addresses but to specific recipients (who have made arrangements for the delivery of incoming mail).

"Non-geographic" postcodes are used for various purposes, many of which were introduced after, or have significantly evolved since, the introduction of the current post code system, including: Admail, "bulk mail" or large volumes (government and business), centralised scanning of inward mail, competitions, parcel returns, direct marketing and PO boxes. They are also referred to as "business service indicator addresses".

Some may be defunct or transferred e.g. Jobcentre typically had the aa98 or aa99 district in each postcode area but each office is now allocated a specific post code in the WV98 and WV99 districts, with the mail digitally scanned after delivery). A limited number are shared with geographic postcode districts, but are differentiated by their post town.

These postcodes cover -

• Post boxes: e.g. in the Brighton BN postcode area, BN50-BN52 are allocated to PO Boxes in Brighton, Rottingdean and Hove.
• Post boxes in London: London postcode districts ending in -P, such as SE1P, are usually allocated to PO Boxes (inconsistently, other letters are also used, and -P has been allocated to geographic addresses, such as SW1P).
• Postcode district: many geographic postcode areas also contain postcode districts that are "non-geographic" and are allocated to specific recipients which are government departments or commercial businesses (e.g. WV98 and WV99 for the Department of Work and Pensions correspondence.[1]). These are referred to as Large User (LU) in the Postcode Address File.
• Postcode area BF: BF is entirely non-geographic (see above).
• Postcode area BX: BX is entirely non-geographic, with all its codes independent of the location of the recipient.
• Postcode area XX: the XX postcode area is non-geographic and is allocated for parcel returns of large volume online retailers (and was used for medical sample testing during the COVID pandemic).

Numbering rules: There appear to be no binding rules publicly available for numbering of non-geographic districts, and therefore can be numbered anywhere in the range 0 to 99, but many such districts are allocated a number higher than the currently existing postcode districts and often in the range 90-99 (several government departments use the AA98 or AA99 district in a number of postcode areas) - retrieved from PAF file, List of postcode areas in the United Kingdom. This is possible in most postcode areas as only a very small number have allocated geographic districts in that number range (B90-B98, BT92-BT94 and M90 seem to be the only geographic districts in this range). However, there are many non-geographic districts numbered outside this range (e.g. American Express has the postcode district BN88; in Glasgow G58 is allocated to National Savings, as part of a mnemonic postcode G58 1SB, though it is located in G43 postcode district).

Significant addressees: Some postcodes in the "non-geographic" range 90–99 are in fact geographic, but specific to the institution or entity and not part of the surrounding numbering sequence (and thus quite different from the neighbouring properties): for example, EH99 1SP can be used with GPS mapping to locate and navigate to the Scottish Parliament (which is directly opposite the Palace of Holyroodhouse, EH8 8DX, and across the road from 7/4 Canongate, EH8 8BX).

Postcode ending nHQ: The letters HQ for the last two letters may also mean it is most likely a non-geographic postcode or that Royal Mail holds the mail where a redirection, bulk mail delivery or open and scan to email service is available.

Girobank's headquarters in Bootle used the non-geographic postcode GIR 0AA.

There is also a special postcode for letters to Santa/Father Christmas, XM4 5HQ.^[49]

Publication: Many non-geographic postcodes do not appear on Royal Mail's own online postcode finder tool or their Click and Drop online postage printing tool, which can add to confusion when responding to organisations that use such addresses. Likewise, delivery services or couriers other than Royal Mail may not be able to deliver to such non-physical addresses. The UK government provides for couriers alternative geographic addresses to their BX addresses.[2]

Extent: Non-geographic postcode districts have been allocated in 63 of the 121 postcode areas (and also in two of three postcode areas of the Crown Dependencies).

Postcodes are allocated by Royal Mail's Address Management Unit. In general, they cannot be purchased or specified by the recipient. Royal Mail has sometimes assigned semi-mnemonic postcodes (sometimes based on the actual geographic postcode district) to high-profile organisations.^[53] Royal Mail does offer a limited form of postcode customisation, so to make it unique and personal. A customised Postcode can only be issued in the UK for a brand new development or an organisation that already has a large user Postcode. This form of customisation is limited to the last two characters (letters) of the postcode.

Prominent examples (for both geographic and special case postcodes) are:

The postcode printed on Business Reply envelopes (which do not require a stamp) often ends with the letters BR.

Post codes ending 1AA are usually allocated to post offices (Crown, main and sub-) such as NR3 1AA for Magdalen Street post office in Norwich. Some are defunct following disposal of former Post Office buildings (e.g. EH1 1AA in Edinburgh).

Postcodes are used to sort letters to their destination either manually, where sorters use labelled frames, or increasingly with letter-coding systems, where machines assist in sorting.^[67] A variation of automated sorting uses optical character recognition (OCR) to read printed postcodes, best suited to mail that uses a standard layout and addressing format.^[68]

A long string of "faced" letters (i.e. turned to allow the address to be read) is presented to a keyboard operator at a coding desk, who types the postcodes onto the envelopes in coloured phosphor dots. The associated machine uses the outward codes in these dots to direct bundles of letters into the correct bags for specific delivery offices. With a machine knowledge of the specific addresses handled by each postal walk at each office, the bundles can be further sorted using the dots of the inward sorting code so that each delivery round receives only its own letters.^[25] This feature depends upon whether it is cost effective to second-sort outward letters, and tends to be used only at main sorting offices where high volumes are handled.^[69]

When postcodes are incomplete or missing, the operator reads the post town name and inserts a code sufficient for outward sorting to the post town, where others can further direct it. The mail bags of letter bundles are sent by road, air or train, and eventually by road to the delivery office.^[69] At the delivery office the mail that is handled manually is inward sorted to the postal walk that will deliver it; it is then "set in", i.e. sorted into the walk order that allows the deliverer the most convenient progress in the round.^[25][69] The latter process is now being automated, as the roll-out of walk sequencing machines continues.^[70][71]

Integrated Mail Processors (IMPs) read the postcode on the item and translate it into two phosphorus barcodes representing the inward and outward parts of the postcode, which the machines subsequently print and read to sort the mail to the correct outward postcode. Letters may also be sequentially sorted by a Compact Sequence Sorter (CSS) reading the outward postcode in the order that a walking postman/woman will deliver, door to door. On such items the top phosphorous barcode is the inward part of the code, the bottom is the outward.^{[citation needed]}

IMPs can also read RM4SCC items, as used in Cleanmail, a different format to the above.

A newer system of five-digit codes called Mailsort was designed for users who send "a minimum of 4,000 letter-sized items".^[72] It encodes the outward part of the postcode in a way that is useful for mail routing, so that a particular range of Mailsort codes goes on a particular plane or lorry. Mailsort users are supplied with a database to allow them to convert from postcodes to Mailsort codes and receive a discount if they deliver mail to the post office split up by Mailsort code. Users providing outgoing mail sorted by postcode receive no such incentive since postcode areas and districts are assigned using permanent mnemonics and do not therefore assist with grouping items together into operationally significant blocks. Walksort^{[clarification needed]} was discontinued in May 2012.

There are approximately 1.7 million postcodes in the United Kingdom, the Channel Islands and the Isle of Man.^[73]

Each postcode is divided by a space into two parts. As mentioned above, the first part starts with the postcode area and ends with the postcode district. The second part begins with a single digit, which indicates the postcode sector, and ends with the postcode unit.

Postcode areas are also divided into several post towns, which predate the introduction of postcodes, with the London post town uniquely covering more than one postcode area.

As of June 2016, there are 124 postcode areas, 2,987 postcode districts, 11,192 postcode sectors, and 1,500 post towns.^[73] As of January 2021^[update], 55,540 full postcodes in England and Wales contain only one household.^[74] Addresses receiving large volumes of mail are each assigned separate "large user" postcodes. But most postcodes are shared by several neighbouring properties, typically covering about 15 addresses.

There are also significant numbers of discontinued (terminated) codes.^[75] Each month some 2,750 postcodes are created and 2,500 terminated.^[76]

The Address Management Unit of Royal Mail maintains an official database of UK postal addresses and postcodes in its Postcode Address File (PAF), which is made available under licence for a fee regulated by Ofcom. The PAF is commercially licensable and is often incorporated in address management software packages. The capabilities of such packages allow most addresses to be constructed solely from the postcode and house number. By including the map references of postcodes in the address database, the postcode can be used to pinpoint a postcode area on a map. PAF is updated daily.

On its website, Royal Mail publishes summary information about major changes to postcode sectors and postal localities (including post towns). Individual postcodes or postal addresses can be found using Royal Mail's Postcode and Address Finder website, but this is limited to 50 free searches per user per day.

A complete list of all current Great Britain postcodes, known as Code-Point Open, has been made available online (since 1 April 2010) by Ordnance Survey. Under the government's OS OpenData initiative, it is available for re-use without charge under an attribution-only licence. The Code-Point Open list includes median coordinates for each postcode but excludes postcodes in Northern Ireland and the Crown dependencies. Unlike the PAF products provided by Royal Mail, the Code-Point Open list does not include postal address text.

The Office for National Statistics also produces postcode directories, under similar licence terms to the OS product. Both the ONSPD and NSPL contain Northern Ireland postcodes, with centroid coordinates in the OSI grid as opposed to the OSGB grid, although Northern Ireland postcodes are subject to a more restrictive licence permitting internal business use only.^[80] Postcodes for the Crown Dependencies are also included, without co-ordinates. A further difference is that non-current postcodes and dates of introduction and withdrawal of postcodes are included.

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.^[81] A businessman in Ilford wishes to have the postcode district of IG1 changed to E19 as he claims customers do not realise his business is based in Greater London.^{[82][importance?]}

Some residents of West Heath in SE2 asked to have their postcodes changed to that of adjacent Bexleyheath, citing higher insurance premiums as reason to change.^[83] Some residents of Kingston Vale in SW15 wish to have their postcodes changed to adjacent Kingston upon Thames for the same reasons^{[citation needed]}.

In all these cases Royal Mail has said that there is "virtually no hope" of changing the postcode, referring to their policy of changing postcodes only to match changes in their operations.^[84] 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.^[85]

Some postcode areas straddle England's borders with Wales and Scotland. Examples of such postcodes include CH4, 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 call signs.

While postcodes were introduced to expedite the delivery of mail, they are useful tools for other purposes, particularly because codes are very fine-grained and identify just a few addresses. Among these uses are:

• With satellite navigation systems, to navigate to an address by street number and postcode
• By life insurance companies and pension funds to assess longevity for pricing and reserving^[86]
• By other types of insurance companies to assess premiums for motoring/business/domestic policies
• To determine catchment areas for school places or doctors' surgeries
• Finding the nearest branch of an organisation to a given address. A computer program uses the postcodes of the target address and the branches to list the closest branches in order of distance as the crow flies (or, if used in conjunction with street-map software, by road distance). This can be used by companies to inform potential customers where to go, by jobcentres to find jobs for job-seekers, to alert people of town planning applications in their area, and a great many other applications.^[22]

The phrase "postcode lottery" refers to the variation in the availability of services by region, though not always because of postcodes.

For these and related reasons, postcodes in some areas have become indicators of social status. Some residents have campaigned to change their postcode to associate themselves with a more desirable area,^[87] to disassociate with a poorer area,^[88] to reduce insurance premiums or to be associated with an area with a lower cost of living.^[89] In all these cases Royal Mail has said that there is "virtually no hope" of changing the postcode, referring to their policy of changing postcodes only to match changes in their operations.^[90]

Postcode areas rarely align with local government boundaries. The phenomenon whereby postcodes overlap administrative boundaries is known as 'straddling'.^[91] Some postcodes straddle England's borders with Wales and Scotland, such as CH1 4QJ^[a] and DG14 0TF.^[b] This has led to British Sky Broadcasting subscribers receiving the wrong BBC and ITV regions, and newly licensed radio amateurs being given incorrect call signs.

• List of postcode areas in the United Kingdom
• List of postal codes (around the world)
• ACORN
• Address Point
• Postcode lottery
• RM4SCC—a machine-readable barcode version of the postcode and delivery point suffix
• Postcode Address File
• Unique Property Reference Number