The currency used as the reference or second currency in a currency pair. When viewing an ISO currency code the counter currency is listed after the base currency, and is separated with a slash. Major currencies, such as the euro and U.S. dollar, are more likely to be the base currency in a currency pair. 




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It is important to understand the structure of the currency pair in order to understand forex trading. In the currency pair USD/EUR the euro (EUR) is considered the counter currency. In the pair, units of counter currency are per unit of base currency.