The essential business of banking involves granting bank deposit credits or issuing IOUs in exchange for deposits (which are claims to base money, such as coins or fiat paper money); banks then use the base money—or that part of it not needed as cash reserves—to purchase other IOUs with the goal of earning a profit on that investment. The business may be most readily understood by considering the elements of a simplified bank balance sheet, where a bank’s available resources—its “assets”—are reckoned alongside its obligations, or “liabilities.”

Bank assets consist mainly of various kinds of loans and marketable securities and of reserves of base money, which may be held either as actual central bank notes and coins or in the form of a credit (deposit) balance at the central bank. The bank’s main liabilities are its capital (including cash reserves and, often, subordinated debt) and deposits. The latter may be from domestic or foreign sources (corporations and firms, private individuals, other banks, and even governments). They may be repayable on demand (sight deposits or current accounts) or after a period of time (time, term, or fixed deposits and, occasionally, savings deposits). The bank’s assets include cash; investments or securities; loans and advances made to customers of all kinds, though primarily to corporations (including term loans and mortgages); and, finally, the bank’s premises, furniture, and fittings.

The difference between the fair market value of a bank’s assets and the book value of its outstanding liabilities represents the bank’s net worth. A bank lacking positive net worth is said to be “insolvent,” and it generally cannot remain open unless it is kept afloat by means of central bank support. At all times a bank must maintain cash balances to pay its depositors upon demand. It must also keep a proportion of its assets in forms that can readily be converted into cash. Only in this way can confidence in the banking system be maintained.

The main resource of a modern bank is borrowed money (that is, deposits), which the bank loans out as profitably as is prudent. Banks also hold cash reserves for interbank settlements as well as to provide depositors with cash on demand, thereby maintaining a “safe” ratio of cash to deposits. The safe cash-to-assets ratio may be established by convention or by statute. If a minimum cash ratio is required by law, a portion of a bank’s assets is in effect frozen and not available to meet sudden demands for cash from the bank’s customers (though the requirement can be enforced in such a way as to allow banks to dip into required reserves on occasion—e.g., by substituting “lagged” for “contemporaneous” reserve accounting). To provide more flexibility, required ratios are frequently based on the average of cash holdings over a specified period, such as a week or a month.

Unless a bank held cash equivalent to 100 percent of its demand deposits, it could not meet the claims of depositors were they all to exercise in full and at the same time their right to demand cash. If that were a common phenomenon, deposit banking could not survive. For the most part, however, the public is prepared to leave its surplus funds on deposit with banks, confident that money will be available when needed. But there may be times when unexpected demands for cash exceed what might reasonably have been anticipated; therefore, a bank must not only hold part of its assets in cash but also must keep a proportion of the remainder in assets that can be quickly converted into cash without significant loss.