A bank note is a promissory note issued by a bank. It is payable on demand, sometimes in the form of precious metals like gold and silver, and sometimes in exchange for assets such as bonds issued by the bank. Bank notes are considered legal tender, and they are used in billions of financial transactions all over the world every day. A bank note, in other words, is currency or money.
Originally, people paid for goods and services with objects of physical value, such as precious metals. Over time, governments began issuing bank notes, pieces of paper which could be exchanged for these objects of value. Although the paper itself is physically worthless, it represents value. These certificates were much lighter and more practical to carry around than piles of gold and silver. Small denominations of currency were cast in coins made from less precious metals, so people generally carried around a mixture of bank notes in paper and small coin.
Over time, the use of a gold or silver standard to back up bank notes was abandoned. This means that a bank note in the United States, for example, can no longer be exchanged with a bank for gold or silver, but it is still convertible into other financially valuable assets. This is known as financial convertibility, differentiating it from physical convertibility. Money which is not physically convertible is still valuable as long as banks have the assets to support it.
In the United States, the Federal Reserve Bank regulates the amount of currency that is made and distributed by the Bureau of Printing and Engraving. All bank notes in the United States have emblems indicating that they are issued by the Federal Reserve. The values of these bank notes are also clearly printed, and they include security features to reduce the risk of forging. Banknotes from other nations are issued by their respective central banks.
Some people collect historical examples of paper money, or banknotes from other countries because they are objects of interest. This is a branch of numismatics, the study of money, currency, and financial systems. In some cases, a bank note may no longer be considered legal currency, but it may have acquired numismatic value. A Confederate bank note from the American Civil War, for example, is not technically money, but is has monetary value because it is an object of interest. People may also establish collections of antique coins in addition to paper money.