U.S. Bureau of Engraving and Printing

Before 1861, the United States did not have a national currency. Instead, the money supply consisted of U.S. coins and a variety of paper notes issued by different private banks. The Federal Government began issuing its own paper currencies in the first years of the Civil War in order to pay for troops and supplies needed for the war effort. Theses early national currencies included Demand Notes, Fractional Currency, United States Notes, National Bank Notes, and Gold Certificates. The latter three continued to be used into the 20th century.

Today, Federal Reserve Notes, produced in denominations of $1, $2, $5, $10, $20, $50 and $100, are the only type of paper currency issued in the United States. However, U.S. currency was not always limited to this single type of note. Not so long ago, a person's wallet might hold several different kinds of paper currency. The design of Current currencies retains three of the most important security features that were first introduced in the 1990s and are easy to check: color-shifting ink, watermark, and security thread. Also, bills are crowded with numbers and letter that help the U.S. Treasury track printing errors and authenticate currency. Serial number is one of the typical examples.

-CL