In forex, prices are quoted as currency pairs: one currency is the base (shown first) and the other is the quote (shown second). A pair tells you how much of the quote currency is needed to buy one unit of the base currency (for example, EUR/USD = 1.10 means €1 costs $1.10). Currencies use three-letter ISO 4217 codes such as EUR, USD, JPY and GBP to keep naming consistent across venues and brokers.