The reserve currency is the main currency that is used by many governments and institutions as foreign reserves. Also it is used to price most commodities. This first graph shows the reserve currencies throughout history, by country. Currently the dollar is the reserve currency. Before that the British pound was the most used currency in the world.
This graph shows the names of the reserve currencies throughout history. From the aureus coin in the first few centuries after Christ. The gulden being the dominant currency in the 17th century. It is not clear to me that the two graphs seem to have interchanged Spain and France. Who knows the correct order?
Now is the question: which currency will follow the dollar?