Central banks implement a country's chosen monetary policy. At the most basic level, this involves establishing what form of currency the country may have, whether a fiat currency, gold-backed currency (disallowed for countries with membership of the International Monetary Fund), currency board or a currency union. When a country has its own national currency, this involves the issue of some form of standardized currency, which is essentially a form of promissory note: a promise to exchange the note for "money" under certain circumstances. Historically, this was often a promise to exchange the money for precious metals in some fixed amount. Now, when many currencies are fiat money, the "promise to pay" consists of the promise to accept that currency to pay for taxes.
A central bank may use another country's currency either directly (in a currency union), or indirectly (a currency board). In the latter case, exemplified by Bulgaria, Hong Kong and Latvia, the local currency is backed at a fixed rate by the central bank's holdings of a foreign currency.
The expression "monetary policy" may also refer more narrowly to the interest-rate targets and other active measures undertaken by the monetary authority.
Frictional unemployment is the time period between jobs when a worker is searching for, or transitioning from one job to another. Unemployment beyond frictional unemployment is classified as unintended unemployment.
For example, structural unemployment is a form of unemployment resulting from a mismatch between demand in the labour market and the skills and locations of the workers seeking employment. Macroeconomic policy generally aims to reduce unintended unemployment.
Keynes labeled any jobs that would be created by a rise in wage-goods (i.e., a decrease in real-wages) as involuntary unemployment:
Men are involuntarily unemployed if, in the event of a small rise in the price of wage-goods relatively to the money-wage, both the aggregate supply of labour willing to work for the current money-wage and the aggregate demand for it at that wage would be greater than the existing volume of employment.
—John Maynard Keynes, The General Theory of Employment, Interest and Money p11
Although the perception by the public may be that the "central bank" controls some or all interest rates and currency rates, economic theory (and substantial empirical evidence) shows that it is impossible to do both at once in an open economy. Robert Mundell's "impossible trinity" is the most famous formulation of these limited powers, and postulates that it is impossible to target monetary policy (broadly, interest rates), the exchange rate (through a fixed rate) and maintain free capital movement. Since most Western economies are now considered "open" with free capital movement, this essentially means that central banks may target interest rates or exchange rates with credibility, but not both at once.
In the most famous case of policy failure, Black Wednesday, George Soros arbitraged the pound sterling's relationship to the ECU and (after making $2 billion himself and forcing the UK to spend over $8bn defending the pound) forced it to abandon its policy. Since then he has been a harsh critic of clumsy bank policies and argued that no one should be able to do what he did.
The most complex relationships are those between the yuan and the US dollar, and between the euro and its neighbours. The situation in Cuba is so exceptional as to require the Cuban peso to be dealt with simply as an exception, since the United States forbids direct trade with Cuba. US dollars were ubiquitous in Cuba's economy after its legalization in 1991, but were officially removed from circulation in 2004 and replaced by the convertible peso.
The main monetary policy instruments available to central banks are open market operation, bank reserve requirement, interest rate policy, re-lending and re-discount (including using the term repurchase market), and credit policy (often coordinated with trade policy). While capital adequacy is important, it is defined and regulated by the Bank for International Settlements, and central banks in practice generally do not apply stricter rules.
To enable open market operations, a central bank must hold foreign exchange reserves (usually in the form of government bonds) and official gold reserves. It will often have some influence over any official or mandated exchange rates: Some exchange rates are managed, some are market based (free float) and many are somewhere in between ("managed float" or "dirty float").
By far the most visible and obvious power of many modern central banks is to influence market interest rates; contrary to popular belief, they rarely "set" rates to a fixed number. Although the mechanism differs from country to country, most use a similar mechanism based on a central bank's ability to create as much fiat money as required.