Exchange control restricts the right of holders of a currency to exchange it for other currencies. It thereby renders a currency inconvertible.

外汇管制是对一国货币的持有者将其兑换成他国货币的限制，因此表现为货币的不可兑换。

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IMF, which knows an inconvertible currency when it sees one, pays them solemn respect.

ECONOMIST: Global poverty

There would be nothing like the same confidence, by the public generally, in an inconvertible currency so regulated, as in a convertible one: and the most instructed person might reasonably doubt whether such a rule would be as likely to be inflexibly adhered to.

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The Indian currency, the rupee, was inconvertible and high tariffs and import licensing prevented foreign goods reaching the market.

BBC: India: the economy