The first two letters of the code are the two letters of ISO 3166-1 alpha-2 country codes (which are also used as the basis for national top-level domains on the Internet) and the third is usually the initial of the currency itself. So Japan's currency code is JPY—JP for Japan and Y for yen. This eliminates the problem caused by the names dollar, franc and pound being used in dozens of different countries, each having significantly differing values. Also, if a currency is revalued, the currency code's last letter is changed to distinguish it from the old currency. In some cases, the third letter is the initial for "new" in that country's language, to distinguish it from an older currency that was revalued; the code sometimes outlasts the usage of the term "new" itself (for example, the code for the Mexican peso is MXN). Other changes can be seen, however; the Russian ruble, for example, changed from RUR to RUB, where the B comes from the third letter in the word "ruble".
There is also a three-digit code number assigned to each currency, in the same manner as there is also a three-digit code number assigned to each country as part of ISO 3166. This numeric code is usually the same as the ISO 3166-1 numeric code. For example, USD (United States dollar) has code 840 which is also the numeric code for the US (United States).
The standard also defines the relationship between the major currency unit and any minor currency unit. Often, the minor currency unit has a value that is 1/100 of the major unit, but 1/1000 is also common. Some currencies do not have any minor currency unit at all. In others, the major currency unit has so little value that the minor unit is no longer generally used (e.g. the Japanese sen, 1/100th of a yen). This is indicated in the standard by the currency exponent. For example, USD has exponent 2, while JPY has exponent 0. Mauritania does not use a decimal division of units, setting 1 ouguiya (UM) = 5 khoums, and Madagascar has 1 ariary = 5 iraimbilanja.
ISO 4217 includes codes not only for currencies, but also for precious metals (gold, silver, palladium and platinum; by definition expressed per one troy ounce, as compared to "1 USD") and certain other entities used in international finance, e.g. Special Drawing Rights. There are also special codes allocated for testing purposes (XTS), and to indicate no currency transactions (XXX). These codes all begin with the letter "X". The precious metals use "X" plus the metal's chemical symbol; silver, for example, is XAG. ISO 3166 never assigns country codes beginning with "X", these codes being assigned for privately customized use only (reserved, never for official codes)—for instance, the ISO 3166-based NATO country codes (STANAG 1059, 9th edition) use "X" codes for imaginary exercise countries ranging from XXB for "Brownland" to XXR for "Redland", as well as for major commands such as XXE for SHAPE or XXS for SACLANT. Consequently, ISO 4217 can use "X" codes for non-country-specific currencies without risk of clashing with future country codes.
Supranational currencies, such as the East Caribbean dollar, the CFP franc, the CFA franc BEAC and the CFA franc BCEAO are normally also represented by codes beginning with an "X". The euro is represented by the code EUR (EU is included in the ISO 3166-1 reserved codes list to represent the European Union). The predecessor to the euro, the European Currency Unit (ECU), had the code XEU.