The most commonly traded currency pairs are made from the most common and actively traded currencies which are called the “Majors”.
When quoting currency pairs, the first currency is referred to as the Base currency while the second referred to as the Counter or Quote currency. The currency pair is used to represent how much Quote currency is required to exchange for the base currency. In a direct quote, the quote currency is the foreign currency.
Example: EUR/USD 1.3500 mean that one Euro is traded for 1.35 USD. As such the Base currency is always equal to 1 monetary unit of exchange. The dominant base currencies are, in order of frequency, the EUR, GBP, and USD. When a currency is quoted against the US Dollar it is called a direct rate. Any currency pair that does not trade against the US Dollar is referred to as a cross rate.
So what takes place once a trade is taking place?
Example: you buy British Pounds with the US Dollars – (GBP/USD), anticipating, the Pound to increase in value relative to the Dollar. If the Pound rises relative to the Dollar, you sell the position (you Sell British Pound) and have made a profit.
Keep in mind that there are no standard cross-currency Quotes. Some have the base currency on the top while others have it on the bottom. So how can you tell which is which? You need to know at least one pair of currencies and which one of the pair is the more valuable.