The currency used is based on the concept of lex monetae; that a sovereign state decides which currency it shall use. The International Organization for Standardization has introduced a system of three-letter codes (ISO 4217) to denote currency (as opposed to simple names or currency signs), in order to remove the confusion arising because there are dozens of currencies called the dollar and several called the franc. Even the "pound" is used in nearly a dozen different countries; most of these are tied to the pound sterling, while the remainder has varying values. In general, the three-letter code uses the ISO 3166-1 country code for the first two letters and the first letter of the name of the currency (D for dollar, for example) as the third letter. United States currency, for instance, is globally referred to as USD. Currencies such as the pound sterling have different codes, as the first two letters denote not the exact country name but an alternative name also used to describe the country. The pound's code is GBP where "GB" denotes Great Britain instead of the United Kingdom.