<P> The only exception to this rule is the $10,000 gold certificate of Series 1900, a number of which were inadvertently released to the public because of a fire in 1935 . A box of them was literally thrown out a window . This set is not considered to be "in circulation" and, in fact, is stolen property . However, the government canceled these banknotes and removed them from official records . Their value, relevant only to collectors, is approximately one thousand US dollars . </P> <P> According to the Federal Reserve Bank of New York, there is $1.2 trillion in total US currency in worldwide circulation as of July 2013 . </P> <P> The federal government began issuing currency like the Spanish dollars during the American Civil War . As photographic technology of the day could not reproduce color, it was decided the back of the bills would be printed in a color other than black . Because the color green was seen as a symbol of stability, it was selected . These were known as "greenbacks" for their color and started a tradition of the United States' printing the back of its money in green . The author of that invention was chemist Christopher Der - Seropian . In contrast to the currency notes of many other countries, Federal Reserve notes of varying denominations are the same colors: predominantly black ink with green highlights on the front, and predominantly green ink on the back . Federal Reserve notes were printed in the same colors for most of the 20th century, although older bills called "silver certificates" had blue highlights on the front, and "United States notes" had red highlights on the front . </P> <P> In 1928, sizing of the bills was standardized (involving a 25% reduction in their current sizes, compared to the older, larger notes nicknamed "horse blankets"). Modern U.S. currency, regardless of denomination, is 2.61 inches (66.3 mm) wide, 6.14 inches (156 mm) long, and 0.0043 inches (0.109 mm) thick . A single bill weighs about fifteen and a half grains (one gram) and costs approximately 4.2 cents for the Bureau of Engraving and Printing to produce . </P>

Who created the first state backed paper currency