One of the most basic concepts of forex trading are currency pairs. Currency trading is done in pairs. A currency pair is the quotation of the relative value of a currency unit against the unit of another currency in the foreign exchange market. The currency that is used as the reference is called the counter currency and the currency that is quoted in relation is called the base currency or transaction currency.
Each currency can be paired with any other, with either capable of being the quoted or base currency, because every currency can be traded.