Forex currencies are always traded in pairs. For example, EUR/USD, which means Euro over US dollars, would be a typical pair. In this case, the Euro, being the first currency can be called the base currency. The second currency, by default USD, is called the counter or quote currency. As mentioned, the first currency is the base, therefore in a pair you can refer the amount of that currency as being the amount required to purchase one unit of the second currency. So, if you want to buy the currency pair, you have to buy the EURO and sell the USD simultaneously. On the other hand, if you are looking to sell the currency pair, you have to sell the EURO and buy the USD. As a part of forex trading strategies the most important thing is to understand the currency pairs, or more precisely in a Forex transaction, what currency you will be selling or buying. Having good knowledge of major currencies of the world is important while learning forex trading.