Money serves three basic functions. By definition, it is a medium of exchange. It also serves as a unit of account and as a store of value—as the “mack” did in Lompoc.

Although money can take an extraordinary variety of forms, there are really only two types of money: money that has intrinsic value and money that does not have intrinsic value.

Commodity moneycommodity moneyMoney that has value apart from its use as money. is money that has value apart from its use as money. Mackerel in federal prisons is an example of commodity money. Mackerel could be used to buy services from other prisoners; they could also be eaten.

Gold and silver are the most widely used forms of commodity money. Gold and silver can be used as jewelry and for some industrial and medicinal purposes, so they have value apart from their use as money. The first known use of gold and silver coins was in the Greek city-state of Lydia in the beginning of the seventh century B.C. The coins were fashioned from electrum, a natural mixture of gold and silver.

One disadvantage of commodity money is that its quantity can fluctuate erratically. Gold, for example, was one form of money in the United States in the 19th century. Gold discoveries in California and later in Alaska sent the quantity of money soaring. Some of this nation’s worst bouts of inflation were set off by increases in the quantity of gold in circulation during the 19th century. A much greater problem exists with commodity money that can be produced. In the southern part of colonial America, for example, tobacco served as money. There was a continuing problem of farmers increasing the quantity of money by growing more tobacco. The problem was sufficiently serious that vigilante squads were organized. They roamed the countryside burning tobacco fields in an effort to keep the quantity of tobacco, hence money, under control. (Remarkably, these squads sought to control the money supply by burning tobacco grown by other farmers.)

Another problem is that commodity money may vary in quality. Given that variability, there is a tendency for lower-quality commodities to drive higher-quality commodities out of circulation. Horses, for example, served as money in colonial New England. It was common for loan obligations to be stated in terms of a quantity of horses to be paid back. Given such obligations, there was a tendency to use lower-quality horses to pay back debts; higher-quality horses were kept out of circulation for other uses. Laws were passed forbidding the use of lame horses in the payment of debts. This is an example of Gresham’s law: the tendency for a lower-quality commodity (bad money) to drive a higher-quality commodity (good money) out of circulation. Unless a means can be found to control the quality of commodity money, the tendency for that quality to decline can threaten its acceptability as a medium of exchange.

But something need not have intrinsic value to serve as money. Fiat moneyfiat moneyMoney that some authority, generally a government, has ordered to be accepted as a medium of exchange. is money that some authority, generally a government, has ordered to be accepted as a medium of exchange. The currencycurrencyPaper money and coins.—paper money and coins—used in the United States today is fiat money; it has no value other than its use as money. You will notice that statement printed on each bill: “This note is legal tender for all debts, public and private.”

Checkable depositscheckable depositsBalances in checking accounts., which are balances in checking accounts, and traveler’s checks are other forms of money that have no intrinsic value. They can be converted to currency, but generally they are not; they simply serve as a medium of exchange. If you want to buy something, you can often pay with a check or a debit card. A checkcheckA written order to a bank to transfer ownership of a checkable deposit. is a written order to a bank to transfer ownership of a checkable deposit. A debit card is the electronic equivalent of a check. Suppose, for example, that you have $100 in your checking account and you write a check to your campus bookstore for $30 or instruct the clerk to swipe your debit card and “charge” it $30. In either case, $30 will be transferred from your checking account to the bookstore’s checking account. Notice that it is the checkable deposit, not the check or debit card, that is money. The check or debit card just tells a bank to transfer money, in this case checkable deposits, from one account to another.

What makes something money is really found in its acceptability, not in whether or not it has intrinsic value or whether or not a government has declared it as such. For example, fiat money tends to be accepted so long as too much of it is not printed too quickly. When that happens, as it did in Russia in the 1990s, people tend to look for other items to serve as money. In the case of Russia, the U.S. dollar became a popular form of money, even though the Russian government still declared the ruble to be its fiat money.

The total quantity of money in the economy at any one time is called the money supplymoney supplyThe total quantity of money in the economy at any one time.. Economists measure the money supply because it affects economic activity. What should be included in the money supply? We want to include as part of the money supply those things that serve as media of exchange. However, the items that provide this function have varied over time.

Before 1980, the basic money supply was measured as the sum of currency in circulation, traveler’s checks, and checkable deposits. Currency serves the medium-of-exchange function very nicely but denies people any interest earnings. (Checking accounts did not earn interest before 1980.)

Over the last few decades, especially as a result of high interest rates and high inflation in the late 1970s, people sought and found ways of holding their financial assets in ways that earn interest and that can easily be converted to money. For example, it is now possible to transfer money from your savings account to your checking account using an automated teller machine (ATM), and then to withdraw cash from your checking account. Thus, many types of savings accounts are easily converted into currency.

Economists refer to the ease with which an asset can be converted into currency as the asset’s liquidityliquidityThe ease with which an asset can be converted into currency.. Currency itself is perfectly liquid; you can always change two $5 bills for a $10 bill. Checkable deposits are almost perfectly liquid; you can easily cash a check or visit an ATM. An office building, however, is highly illiquid. It can be converted to money only by selling it, a time-consuming and costly process.

As financial assets other than checkable deposits have become more liquid, economists have had to develop broader measures of money that would correspond to economic activity. In the United States, the final arbiter of what is and what is not measured as money is the Federal Reserve System. Because it is difficult to determine what (and what not) to measure as money, the Fed reports several different measures of money, including M1 and M2.

M1M1The narrowest of the Fed’s money supply definitions that includes currency in circulation, checkable deposits, and traveler’s checks. is the narrowest of the Fed’s money supply definitions. It includes currency in circulation, checkable deposits, and traveler’s checks. M2M2A broader measure of the money supply than M1 that includes M1 and other deposits. is a broader measure of the money supply than M1. It includes M1 and other deposits such as small savings accounts (less than $100,000), as well as accounts such as money market mutual funds (MMMFs) that place limits on the number or the amounts of the checks that can be written in a certain period.

With all the operational definitions of money available, which one should we use? Economists generally answer that question by asking another: Which measure of money is most closely related to real GDP and the price level? As that changes, so must the definition of money.

In 1980, the Fed decided that changes in the ways people were managing their money made M1 useless for policy choices. Indeed, the Fed now pays little attention to M2 either. It has largely given up tracking a particular measure of the money supply. The choice of what to measure as money remains the subject of continuing research and considerable debate.