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Even for narrow aggregates like M1, by far the largest part of the money supply consists of deposits in commercial banks, whereas currency (banknotes and coins) issued by central banks only makes up a small part of the total money supply in modern economies. The public's demand for currency and bank deposits and commercial banks' supply of loans are consequently important determinants of money supply changes. As these decisions are influenced by central banks' monetary policy, not least their setting of interest rates, the money supply is ultimately determined by complex interactions between non-banks, commercial banks and central banks.

According to the quantity theory supported by the monetarist school of thought, there is a tight causal connection between growth in the money supply and inflation. In particular during the 1970s and 1980s this idea was influential, and several major central banks during that period attempted to control the money supply closely, following a monetary policy target of increasing the money supply stably.

Consequently, the money supply has lost its central role in monetary policy, and central banks today generally do not try to control the money supply. Instead they focus on adjusting interest rates, in developed countries normally as part of a direct inflation target which leaves little room for a special emphasis on the money supply. Money supply measures may still play a role in monetary policy, however, as one of many economic indicators that central bankers monitor to judge likely future movements in central variables like employment and inflation.

Measures of money supply

There are several standard measures of the money supply, classified along a spectrum or continuum between narrow and broad monetary aggregates. Narrow measures include only the most liquid assets: those most easily used to spend (currency, checkable deposits). Broader measures add less liquid types of assets (certificates of deposit, etc.).

This continuum corresponds to the way that different types of money are more or less controlled by monetary policy. Narrow measures include those more directly affected and controlled by monetary policy, whereas broader measures are less closely related to monetary-policy actions.

The different types of money are typically classified as "M"s. The "M"s usually range from M0 (narrowest) to M3 (and M4 in some countries) (broadest), but which "M"s, if any, are actually focused on in central bank communications depends on the particular institution. A typical layout for each of the "M"s is as follows for the United States:

  • M0: In some countries, such as the United Kingdom, M0 includes bank reserves, so M0 is referred to as the monetary base, or narrow money.
  • MB: is referred to as the monetary base or total currency. This is the base from which other forms of money (like checking deposits, listed below) are created and is traditionally the most liquid measure of the money supply.
  • M1: Bank reserves are not included in M1.
  • M2: Represents M1 and "close substitutes" for M1. M2 is a broader classification of money than M1.
  • M3: M2 plus large and long-term deposits.
  • MZM: Money with zero maturity. It measures the supply of financial assets redeemable at par on demand.

Creation of money

Both central banks and commercial banks play a role in the process of money creation. In short, in the fractional-reserve banking system used throughout the world, money can be subdivided into two types:

  • central bank money – obligations of a central bank, including currency and central bank depository accounts
  • commercial bank money – obligations of commercial banks, including checking accounts and savings accounts.

In the money supply statistics, central bank money is MB while the commercial bank money is divided up into the M1–M3 components, where it makes up the non-M0 component.

By far the largest part of the money used by individuals and firms to execute economic actions are commercial bank money, i.e. deposits issued by banks and other financial institutions. In the United Kingdom, deposit money outweighs the central bank issued currency by a factor of more than 30 to 1. In the United States, where the country's currency has a special international role being used in many transactions around the world, legally as well as illegally, the ratio is still more than 8 to 1. Commercial banks create money whenever they make a loan and simultaneously create a matching deposit in the borrower's bank account. In return, money is destroyed when the borrower pays back the principal on the loan. Movements in the money supply therefore to a large extent depend on the decisions of commercial banks to supply loans and consequently deposits, and the public's behavior in demanding currency as well as bank deposits. These decisions are influenced by the monetary policy of central banks, so that money supply is ultimately created by complex interactions between banks, non-banks and central banks.

Even though central banks today rarely try to control the amount of money in circulation, their policies still impact the actions of both commercial banks and their customers. When setting the interest rate on central bank reserves, interest rates on bank loans are affected, which in turn affects their demand. Central banks may also affect the money supply more directly by engaging in various open market operations. They can increase the money supply by purchasing government securities, such as government bonds or treasury bills. This increases the liquidity in the banking system by converting the illiquid securities of commercial banks into liquid deposits at the central bank. This also causes the price of such securities to rise due to the increased demand, and interest rates to fall. In contrast, when the central bank "tightens" the money supply, it sells securities on the open market, drawing liquid funds out of the banking system. The prices of such securities fall as supply is increased, and interest rates rise.

In some economics textbooks, the supply-demand equilibrium in the markets for money and reserves is represented by a simple so-called money multiplier relationship between the monetary base of the central bank and the resulting money supply including commercial bank deposits. This is a short-hand simplification which disregards several other factors determining commercial banks' reserve-to-deposit ratios and the public's money demand.

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