A system under which holders of a national currency require official permission or approval to convert it into other currencies. Exchange control may apply to all holders of a currency, or some holders, usually non-residents, may be exempt. Where non-residents are allowed convertibility, exchange control may be applied to transfers from resident to non-resident domestic currency bank accounts. Exchange controls may have varying degrees of strictness; frequently they are much more stringent on obtaining foreign currency for capital account than for current account purposes.