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.