A currency pair is the dyadic 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, quote currency, or currency and the currency that is quoted in relation is called the base currency or transaction currency.

Currency pairs are generally written by concatenating the ISO currency codes (ISO 4217) of the base currency and the counter currency, and then separating the two codes with a slash. Alternatively the slash may be omitted, or replaced by either a dot or a dash. A widely traded currency pair is the relation of the euro against the US dollar, designated as EUR/USD. The quotation EUR/USD 1.2500 means that one euro is exchanged for 1.2500 US dollars. Here, EUR is the base currency and USD is the quote currency (counter currency). This means that 1 Euro can be exchangeable to 1.25 US Dollars.

The most traded currency pairs in the world are called the Majors. They involve the currencies euro, US dollar, Japanese yen, pound sterling, Australian dollar, Canadian dollar, and the Swiss franc.