text: The initial postcode system evolved from named postal districts introduced in London and other large cities from 1857. London was divided into numbered subdivisions in 1917, which extended to other cities by 1934. Each postcode area contains multiple post towns and districts. For example, the GU postcode area covers a significant part of southern England, with Guildford as a major town, and Woking as another post town within the GU22 district.
  Overview
  Structure
  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 name 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, but sometimes more).  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).