The US dollar was founded, as the official currency of the United States, with the Coinage Act of 1792. At that time, the Spanish dollar was the most commonly circulated currency. By 1857, the US Dollar became the country’s only legal tender and its official currency. The stability of the US Dollar means that it was the world’s dominant reserve currency from the mid-20th century to the early 21st. In 1968, the US Dollar was changed from a gold standard, where its value was linked to a certain quantity of gold, to a fiat currency, which is not attached to a commodity. Throughout the 20th century, there has been a constant decline in the value of the US Dollar. In 2017, the US Dollar became the official currency of nine countries.