These are the currencies of nations which have a dominant role in global economic transactions. The European Union, Japan, the United States are the important powers the currencies of which fill the coffers of central banks around the world. Among those, the role of the Japanese Yen as a reserve currency has been diminishing since the 90’s, while that of the Euro has been increasing continuously since the launch of the currency. Among all those changes however, the US Dollar has remained as the one major currency that has the greatest preponderance over everything else in central bank currency allocations. With about two thirds of global forex reserves denominated in the dollar, the USD is the reserve currency of the world.
For traders, an important rule of thumb is that reserve currencies as a group tend to depreciate in times of boom, and to appreciate at times of economic trouble. This is a generalization; needless to say there is a degree of variation among the behavior of different currencies, but due to the financial structure of the global economy, economic activity usually leads to abundant supply of reserve currencies during robust economic growth.