<human>: Can you tell me about the history of the U.S. dollar?
<bot>: The United States dollar is the world's most widely circulated currency, with over 40% of global GDP estimated to be held in dollars as of 2017. It was first issued in 1785 by the First Bank of the United States and has been used since then for various transactions such as buying goods and services. Over time, it became the primary currency of international trade and exchange, replacing the Spanish silver dollar as the main unit of account. In 1971, President Nixon took the United States off the gold standard, allowing the dollar to float freely against other currencies. Since then, the value of the US dollar has fluctuated significantly, but overall its purchasing power has remained relatively stable.