Washington Bill is a term used to refer to the United States ONE-DOLLAR bill. It is also known as a 'GREENBACK' due to its green color. The ONE-DOLLAR bill is the most common denomination of paper currency in circulation in the United States and is the first U.S. paper currency to feature a portrait of an American president, George Washington. The ONE-DOLLAR bill is also the only U.S. currency to feature a single letter in its serial number, the letter 'A'. The ONE-DOLLAR bill is also the only U.S. currency to feature a ONE-spot on its reverse side, which is a small circle located in the lower right corner.