Postal codes used in the United Kingdom, British Overseas Territories and Crown dependencies are known as postcodes (originally, postal codes). They are alphanumeric and were adopted nationally between 11 October 1959 and 1974, having been devised by the General Post Office (Royal Mail). A full postcode is known as a "postcode unit" and designates an area with several addresses or a single major delivery point.
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 15 addresses). The incode always has 3 characters, starting with a number (denoting a sector within the district), and ending 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. A small number are geographic in nature — such as HS for the Outer Hebrides and FY for Fylde (the region around Blackpool).
Each postcode area contains a number of post towns which are not themselves alphabetically denoted; however each will generally constitute one or more postcode districts. Example: a sizeable part of southern England is covered by the GU postcode area, named after the town of Guildford. Guildford itself consists of postal districts GU1 and GU2. Nearby Woking, a major commuter town—6 miles (10 km) away—is a post town within the postal district GU22. The central part of the town/city the postcode area is named after will often have the number 1 e.g. B1 (Birmingham), LS1 (Leeds), M1 (Manchester). Other post towns within the area are treated variously alphabetically, particularly in London (e.g. Chingford on the north-eastern edge of London is E4, whereas adjacent Walthamstow to the south is E17), or geographically (e.g. the Outer Hebrides area HS numbers the districts from north to south).
As a general rule, large post towns are numbered from the centre outward such that outlying parts have higher numbered districts. However, the disparate post towns within a postal area can be numbered based on various criteria. However, the centrality of a postcode district within a postcode area cannot be reliably inferred from the postcode alone. For instance, SE1 covers a large part of Central London south of the Thames whereas SE2 covers Abbey Wood at the far end of the Elizabeth Line. 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.
The initial system of named postal districts, developed in London and other large cities from 1857, evolved towards the present form: in 1917 London was split into broad numbered subdivisions, and this extended to the other cities in 1934.
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.