A bank may mobilize its assets in several ways. It may demand repayment of loans, immediately or at short notice; it may sell securities; or it may borrow from the central bank, using paper representing investments or loans as security. Banks do not precipitately call in loans or sell marketable assets, because this would disrupt the delicate debtor-creditor relationship and lessen confidence, which probably would result in a run on the banks. Banks therefore maintain cash reserves and other liquid assets at a certain level or have access to a “lender of last resort,” such as a central bank. In a number of countries, commercial banks have at times been required to maintain a minimum liquid assets ratio. Among the assets of commercial banks, investments are less liquid than money-market assets. By maintaining an appropriate spread of maturities (through a combination of long-term and short-term investments), however, it is possible to ensure that a proportion of a bank’s investments will regularly approach redemption. This produces a steady flow of liquidity and thereby constitutes a secondary liquid assets reserve.

Yet this necessity—to convert a significant portion of its liabilities into cash on demand—forces banks to “borrow short and lend long.” Because most bank loans have definite maturity dates, banks must exchange IOUs that may be redeemed at any time for IOUs that will not come due until some definite future date. That makes even the most solvent banks subject to liquidity risk—that is, the risk of not having enough cash (base money) on hand to meet demands for immediate payment.

Banks manage this liquidity risk in a number of ways. One approach, known as asset management, concentrates on adjusting the composition of the bank’s assets—its portfolio of loans, securities, and cash. This approach exerts little control over the bank’s liabilities and overall size, both of which depend on the number of customers who deposit savings in the bank. In general, bank managers build a portfolio of assets capable of earning the greatest interest revenue possible while keeping risks within acceptable bounds. Bankers must also set aside cash reserves sufficient to meet routine demands (including the demand for reserves to meet minimum statutory requirements) while devoting remaining funds mainly to short-term commercial loans. The presence of many short-term loans among a bank’s assets means that some bank loans are always coming due, making it possible for a bank to meet exceptional cash withdrawals or settlement dues by refraining from renewing or replacing some maturing loans.

The practice among early bankers of focusing on short-term commercial loans, which was understandable given the assets they had to choose from, eventually became the basis for a fallacious theory known as the “real bills doctrine,” according to which there could be no risk of banks overextending themselves or generating inflation as long as they stuck to short-term lending, especially if they limited themselves to discounting commercial bills or promissory notes supposedly representing “real” goods in various stages of production. The real bills doctrine erred in treating both the total value of outstanding commercial bills and the proportion of such bills presented to banks for discounting as being values independent of banking policy (and independent of bank discount and interest rates in particular). According to the real bills doctrine, if such rates are set low enough, the volume of loans and discounts will increase while the outstanding quantity of bank money will expand; in turn, this expansion may cause the general price level to rise. As prices rise, the nominal stock of “real bills” will tend to grow as well. Inflation might therefore continue forever despite strict adherence by banks to the real bills rule.

Although the real bills doctrine continues to command a small following among some contemporary economists, by the late 19th century most bankers had abandoned the practice of limiting themselves to short-term commercial loans, preferring instead to mix such loans with higher-yielding long-term investments. This change stemmed in part from increased transparency and greater efficiency in the market for long-term securities. These improvements have made it easy for an individual bank to find buyers for such securities whenever it seeks to exchange them for cash. Banks also have made greater use of money-market assets such as treasury bills, which combine short maturities with ready marketability and are a favoured form of collateral for central bank loans.

Commercial banks in some countries, including Germany, also make long-term loans to industry (also known as commercial loans) despite the fact that such loans are neither self-liquidating (capable of generating cash) nor readily marketable. These banks must ensure their liquidity by maintaining relatively high levels of capital (including conservatively valued shares in the enterprises they are helping to fund) and by relying more heavily on longer-term borrowings (including time deposits as well as the issuance of bonds or unsecured debt, such as debentures). In other countries, including Japan and the United States, long-term corporate financing is handled primarily by financial institutions that specialize in commercial loans and securities underwriting rather than by banks.