A banknote is a piece of paper that serves as a promissory note of payment. Originally, bank notes served as IOUs. Farmers and merchants handed over pieces of paper to creditors, essentially guaranteeing them something of value. However, thanks to the evolution of banking systems and the increasing sophistication of global financial networks, this kind of buyer use system has become moribund for all intents and purposes.

This is not to say that some countries or districts haven't tried to create alternative banknote systems. One prominent attempt to do so took place in Ithaca, New York. Laborers decided to create something called the Ithaca Hours, which were intended to be backed up by the intrinsic value of the labor force of the town.

U.S. bank notes come in a variety of denominations, such as the familiar $1 bill, $5 bill, $10 bill, $20 bill, $50 bill, and $100 bill. While larger denominations exist in circulation, even people of means rarely employ them in transactions. Indeed, if a cashier clerk spots a bill larger than $100, he or she will likely become suspicious of forgery.

Theoretically, all of the bank notes produced in the United States Treasury are backed up by gold and silver in national vaults. The largest such vault is Fort Knox in Kentucky, which houses the world's largest storehouse of gold bullion. The U.S. Treasury recently changed the design of the banknotes in circulation to prevent increasingly aggressive forgery attempts.