The list of postcode areas in the United Kingdom is a tabulation of the postcode areas used by Royal Mail for the purposes of directing mail within the United Kingdom. The postcode area is the largest geographical unit used and forms the initial characters of the alphanumeric UK postcode. There are currently 121 geographic postcode areas in use in the UK and a further three often combined with these covering the Crown Dependencies of Guernsey, Jersey and Isle of Man. Subdivision Each postcode area is further divided into post towns and postcode districts. There are on average 20 postcode districts to a postcode area. The London post town is instead divided into several postcode areas. Scope The single or pair of letters chosen for postcode areas are generally intended as a mnemonic for the places served. Postcode areas, post towns and postcode districts do not follow political boundaries and usually serve much larger areas than the place names with which they are associated. For example, within the PA postcode area the PA1 and PA78 postcode districts are 140 miles apart; and the eight postcode areas of the London post town cover only 40% of Greater London. The remainder of its area is covered by sections of twelve adjoining postcode areas: EN, IG, RM, DA, BR, TN, CR, SM, KT, TW, HA and UB. United Kingdom postcode areas Crown dependencies The Crown dependencies (which are not part of the United Kingdom) did not introduce postcodes until later, but use a similar coding scheme. They are separate postal authorities. Defunct postcode areas London NE and S Glasgow Glasgow, like London, was divided into compass districts: C, W, NW, N, E, SE, S, SW. When postcodes were introduced, these were mapped into the new G postcode: C1 became G1, W1 became G11, N1 became G21, E1 became G31, S1 became G41, SW1 became G51, and so on. As NW and SE had never been subdivided they became G20 and G40 respectively. Norwich and Croydon Norwich and Croydon were used for a postcode experiment in the late 1960s, which was replaced by the current system. The format was of the form NOR or CRO followed by two numbers and a letter, e.g. NOR 07A. They were later changed to CR0 (digit '0') and NR1. Dublin, Ireland When Ireland was a part of the United Kingdom of Great Britain and Ireland a postal district system