A common term today, The “Greenback” – which denoted a form of United States Currency, was designed and circulated first during The American Civil War of 1861-1865. During the early War, these Monies were known AS “Demand Notes”. The War put huge strains of The Federal Government, who had to rely on it’s Gold and Silver Reserves to fund a massive Army and the materials needed to maintain such. As it (and other Countries had done), it relied on such “Promisory Notes” which could be redeemed (if necessary) in it’s related amount of Reserve. However, “Green Backs” were not based on Federal Reserves only the credibility of The Federal Government. This made it’s usage in payment with other Countries in Europe somewhat problematic (due to a War in which many, at first – thought The United States would lose).
Originally, before The Civil War – “Specie Notes” were used expressly by “Private Banks” as payment on demand. The United States used Gold or Silver Coinage. Most “Paper Money” was not considered “Legal Tender”. In the early 19th Century, Paper Currency was used as “Custom Payment” in Export and Import.
During The War, The Confederate Government in kind, created their own Paper Currency (based on the exact system used by The Federals). However, unlike The North – each State issued it’s own particular Currency.
The name of “Greenbacks” came simply from the color of the Back of the money. Printed in two colors – the front being Gray and the Back was Green – hence the term “Green Back”. Although after The War, Monies for a brief period, went back to a Coinage based System – the style and development of Modern US Paper Currency was born out of the “fires” of War.