All currencies are traded in pairs. The first currency in the pair is called the base currency while the second is called the quote or "counterpart" currency. Usually the most dominant currency, in terms of the other currencies against which it trades, is quoted first. This is a matter of perspective because, from a local point of view, the local currency is the most dominant. One always wants to know just how much of the foreign currency one can get when exchanging the local currency. An American may want to know how many euros he can get for his dollar or, if he was traveling to Japan, how many yen he can get. (Interested in forex but don't know where to start? Check out Top 7 Questions About Currency Trading Answered.)
However, based on international convention among banks, certain currencies have been assigned trading dominance. The euro represents some 17 countries that have joined the euro system and, therefore, has become the dominant base currency against all other global currencies, so it is quoted first. For example, the euro, represented as EUR, will be identified as EUR/USD, EUR/GBP, EUR/CHF, EUR/JPY, EUR/CAD, etc. All have the EUR acronym as the first currency in the sequence.
The British pound, originally the main currency of the world during the heights of the British Empire, is next in the hierarchy of currency name domination. The major currency pairs versus the GBP are identified as GBP/USD, GBP/CHF, GBP/JPY, GBP/CAD. Apart from the EUR/GBP, the GBP is usually the first currency quoted when it is involved in a currency pair. Because the U.S. dollar is the de facto world reserve currency since the end of the Second World War, and because commodity prices such as gold and oil are quoted in dollars, one could argue that the dollar is really the dominant currency of the world. In terms of convention, however, it ranks third in dominance and is quoted first against the Canadian dollar, Japanese yen and Swiss franc. In short, tradition and convention seem to play larger roles in the hierarchy of currency pairs than the relative economic strength of the economies that the currencies represent.(For further reading, see Using Currency Correlations To Your Advantage.)