In 1861, as a means of financing the American Civil War, the federal government began issuing paper money for the first time since the Continental Congress printed currency to help pay for the Revolutionary War (the earlier form of paper dollars, dubbed continentals, were produced in such high volume that they soon lost much of their value). In the decades leading up to the Civil War, private, state-chartered banks printed paper money, resulting in a wide range of denominations and designs. The new bills circulated by the U.S. government starting in the 1860s came to be known as greenbacks because their back sides were printed in green ink. This ink was an anti-counterfeiting measure used to prevent photographic knockoffs, since the cameras of the time could only take pictures in black and white.

In 1929, the government shrunk the size of all paper money (in order to cut down on manufacturing costs) and instituted standardized designs for each denomination, which made it easier for people to tell real bills from fakes. The small-sized bills continued to be printed with green ink because, according to the U.S. Bureau of Printing and Engraving, the ink was plentiful and durable and the color green was associated with stability.

Today, there is some $1.2 trillion in coins and paper money in circulation in America. It costs about 5 cents to produce every $1 bill and around 13 cents to make a $100 bill, the highest denomination currently in circulation. The estimated life span of a $1 bill is close to six years, while a $100 bill typically lasts 15 years. The $50 bill has the shortest average life span: 3.7 years.