A currency code is a short standard identifier for a currency, and nearly all codes in use are those defined by ISO 4217. Each currency has a three-letter alphabetic code, such as USD for the United States dollar and EUR for the euro, and a three-digit numeric code, such as 840 and 978 for the same two. Codes remove the ambiguity of symbols and names: the dollar sign alone could mean any of more than twenty currencies.
In most alphabetic codes, the first two letters are the country code from ISO 3166 and the third is the initial of the currency, so JPY is the Japanese yen and GBP the British pound. Supranational currencies and precious metals use codes beginning with X, as in XAU for gold. The standard also records each currency's minor unit, the number of decimal places, which is two for most currencies, zero for the yen and three for a few such as the Kuwaiti dinar.
Payment systems depend on these codes. Card authorization messages carry the numeric code, bank messages under ISO 20022 carry the alphabetic one, and payment interfaces require a code with every amount. An exchange rate is quoted as a currency pair of two codes.