Internationally, the international standard of the International Organization for Standardization (ISO), namely ISO 42 17 "Code Representation of Money and Capital" (code representing money and capital) is adopted as the currency identification standard to distinguish the currencies of countries or regions with similar foreign currency symbols. Each currency has two kinds of codes, the first is the commonly used three-digit code, and the second is the rarely used three-digit code. Most signs are composed of a unique three-letter code, that is, the two-letter code of ISO 3 166- 1 followed by a letter (usually the first letter of the currency name). For example, GBP stands for pound sterling and USD stands for dollar. In foreign exchange quotation, currency pairs are represented by two ISO codes plus a separator, such as GBP/USD, where the first code represents "basic currency" and the other code represents "auxiliary currency".