To see how banks influence the money supply, it is useful to imagine first a world without any banks at all. In this simple world, currency is the only form of money. To be concrete, let's suppose that the total quantity of currency is $100. The supply of money is, therefore, $100.
Now suppose that someone opens a bank, appropriately called First National Bank. First National Bank is only a depository institution—that is, it accepts deposits but does not make loans. The purpose of the bank is to give depositors a safe place to keep their money. Whenever a person deposits some money, the bank keeps the money in its vault until the depositor comes to withdraw it or writes a check against his or her balance. Deposits that banks have received but have not reserves loaned out are called reserves. In this imaginary economy, all deposits are held as deposits that banks have received but reserves, so this system is called 100-percent-reserve banking.
have not loaned out We can express the financial position of First National Bank with a T-account, which is a simplified accounting statement that shows changes in a bank's assets and liabilities. Here is the T-account for First National Bank if the economy's entire $100 of money is deposited in the bank:
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