A single currency is when a number of countries join their currencies together into one single currency. The main example of this is the introduction of the Euro in the European Union. This was introduced in January 1999 with the full changeover to notes and coins taking place in January 2001. This meant that the Eurozone was made up of 12 countries with three EU members (UK, Denmark and Greece) not joining the single currency. The enlargement of the EU on May 1st 2004 further expanded the Eurozone.
Dollar was chosen to become the monetary unit for the USA in 1785. The Coinage Act of 1792 helped put together an organised monetary system that introduced coinage in gold, silver, and copper. Paper notes or greenbacks were introduced into the system in 1861 to help finance the Civil War
The history of the United States Dollar refers to more than 240 years since the Continental Congress of the United States authorized the issuance of Continental Currency in 1775. On April 2, 1792, the United States Congress created the United States dollar as the country's standard unit of money