U.S. Paper Money

A United States Note, known as a Legal Tender Note, is a type of paper money that was issued from 1862 to 1971. They were known popularly as "greenbacks" in their heyday, a name inherited from the earlier greenbacks, the Demand Notes, that they replaced in 1862. They were originally issued directly into circulation by the U.S. Treasury to pay expenses incurred by the Union during the American Civil War.
Prior to the American Civil War, state banks also issued their own banknotes and chartered private banks to issue banknotes as well.
Federal Reserve Bank Notes, issued 1915–1934, differ from Federal Reserve Notes in that they are backed by one of the twelve Federal Reserve Banks, rather than by all collectively. The authority of the Federal Reserve Banks to issue notes comes from the Federal Reserve Act of 1913. Legally, they are liabilities of the Federal Reserve Banks and obligations of the United States government. Although not issued by the Treasury Department, Federal Reserve Notes carry the (engraved) signature of the Treasurer of the United States and the United States Secretary of the Treasury.
The Emergency Banking Act of 1933 removed the gold obligation and authorized the Treasury to satisfy these redemption demands with current notes of equal face value (effectively making change). Under the Bretton Woods system, although citizens could not possess gold, the federal government continued to maintain a stable international gold price.
This system ended with the Nixon Shock of 1971. Present-day Federal Reserve Notes are not backed by convertibility to any specific commodity, but only by the collateral assets that Federal Reserve Banks post in order to obtain them.