In currency trading, only the most economically/politically stable and liquid currencies are demanded in sufficient quantities. For example, due to the size and strength of the United States economy, the American dollar is the world's most actively traded currency. 

The major currency pairs all contain the US dollar and are the most traded pairs on the Forex market. For this reason they are very liquid.

Over 95% of all daily Forex transactions involve: the U.S dollar (USD), the Euro (EUR), the British pound (GBP), the Japanese yen (JPY), the Swiss franc (CHF), the New Zealand dollar (NZD), the Australian dollar (AUD) and the Canadian dollar (CAD).