What is fractional-reserve banking?
Fractional-reserve banking is the system of banking operating in almost all countries worldwide, under which banks that take deposits from the public are required to hold a proportion of their deposit liabilities in liquid assets as a reserve, and are at liberty to lend the remainder to borrowers. Bank reserves are held as cash in the bank or as balances in the bank's account at the central bank. The country's central bank determines the minimum amount that banks must hold in liquid assets, called the "reserve requirement" or "reserve ratio". Most commercial banks hold more than this minimum amount as excess reserves.

Bank deposits are usually of a relatively short-term duration, and may be "at call", while loans made by banks tend to be longer-term, resulting in a risk that customers may at any time collectively wish to withdraw cash out of their accounts in excess of the bank reserves. The reserves only provide liquidity to cover withdrawals within the normal pattern. Banks and the central bank expect that in normal circumstances only a proportion of deposits will be withdrawn at the same time, and that reserves will be sufficient to meet the demand for cash. However, banks may find themselves in a shortfall situation when depositors wish to withdraw more funds than the reserves held by the bank. In that event, the bank experiencing the liquidity shortfall may borrow short-term funds in the interbank lending market from banks with a surplus. In exceptional situations, such as during an unexpected bank run, the central bank may provide funds to cover the short-term shortfall as lender of last resort.