When doing research for historical books, there are a lot of things that can get complicated fast. One of them is currency or money and how the heck did people pay for what they wanted? Of course, there has always been barter but basically a culture that expects to grow needs a system of dependable currency.
For a long time, the dollars people accepted in payment were printed and stood behind by a multitude of banks-- which incidentally could fail. If you accepted dollars, you might want to know which bank had issued them. For years, the federal government did not issue money. It wasn't until 1862 that it set standards for paper and sizes.
During the Civil War, the South, in leaving the Union, needed its own currency. This was done through various states and of course, became worthless when the South lost the war (except someday to collectors, of course).
With the failure of banks, which happened again and again with various recessions, eventually there came a need for a consistent currency that the federal government stood behind and issued. That came in 1914. At first, it was backed by gold-- the gold standard.
Then along came the Great Depression and a run on banks to get gold. In 1933, Roosevelt took the US off the gold standard. People could no longer redeem their notes for gold and money began to float. That takes us to today when money stays constant... other than inflation that is. Ever check out the price of a paperback book in 1950 something? Try 25¢. Yeah the government stands behind it... way behind it :).
