Currency union

A currency union (also known as monetary union) is an intergovernmental agreement that involves two or more states sharing the same currency. These states may not necessarily have any further integration (such as an economic and monetary union, which would have, in addition, a customs union and a single market).
There are three types of currency unions:
The theory of the optimal currency area addresses the question of how to determine what geographical regions should share a currency in order to maximize economic efficiency.
Implementing a new currency in a country is always a controversial topic because it has both many advantages and disadvantages. New currency has different impacts on businesses and individuals, which creates more points of view on the usefulness of currency unions. As a consequence, governmental institutions often struggle when they try to implement a new currency, for example by entering a currency union.
Convergence in terms of macroeconomics means that countries have a similar economic behaviour (similar inflation rates and economic growth). It is easier to form a currency union for countries with more convergence as these countries have the same or at least very similar goals. The European Monetary Union (EMU) is a contemporary model for forming currency unions. Membership in the EMU requires that countries follow a strictly defined set of criteria (the member states are required to have a specific rate of inflation, government deficit, government debt, long-term interest rates and exchange rate). Many other unions have adopted the view that convergence is necessary, so they now follow similar rules to aim the same direction.
Divergence is the exact opposite of convergence. Countries with different goals are very difficult to integrate in a single currency union. Their economic behaviour is completely different, which may lead to disagreements. Divergence is therefore not optimal for forming a currency union.
The first currency unions were established in the 19th century. The German Zollverein came into existence in 1834, and by 1866, it included most of the German states. The fragmented states of the German Confederation agreed on common policies to increase trade and political unity.
The Latin Monetary Union, comprising France, Belgium, Italy, Switzerland, and Greece, existed between 1865 and 1927, with coinage made of gold and silver. Coins of each country were legal tender and freely interchangeable across the area. The union's success made other states join informally.
The Scandinavian Monetary Union, comprising Sweden, Denmark, and Norway, existed between 1873 and 1905 and used a currency based on gold. The system was dissolved by Sweden in 1924.
A currency union among the British colonies and protectorates in Southeast Asia, namely the Federation of Malaya, North Borneo, Sarawak, Singapore and Brunei was established in 1952. The Malaya and British Borneo dollar, the common currency for circulation was issued by the Board of Commissioners of Currency, Malaya and British Borneo from 1953 until 1967. Following the cessation of the common currency arrangement, Malaysia (the combination of Federation of Malaya, North Borneo, Sarawak), Singapore and Brunei began issuing their own currencies. Contemporarily, a currency reunion of these countries might still be feasible based on the findings of economic convergence.
Note: Every customs and monetary union and economic and monetary union also has a currency union.
Zimbabwe is theoretically in a currency union with four blocs as the South African rand, Botswana pula, British pound and US dollar freely circulate. The US Dollar was, until 2016, official tender.
Additionally, the autonomous and dependent territories, such as some of the EU member state special territories, are sometimes treated as separate customs territory from their mainland state or have varying arrangements of formal or de facto customs union, common market and currency union (or combinations thereof) with the mainland and in regards to third countries through the trade pacts signed by the mainland state.
The European currency union is a part of the Economic and Monetary Union of the European Union (EMU). EMU was formed during the second half of the 20th century after historic agreements, such as Treaty of Paris (1951), Maastricht Treaty (1992). In 2002, the euro, a single European currency, was adopted by 12 member states. Currently, the Eurozone has 20 member states. The other members of the European Union are required to adopt the euro as their currency (except for Denmark, which has been given the right to opt out), but there has not been a specific date set. The main independent institution responsible for stability of the euro is the European Central Bank (ECB). The Eurosystem groups together the ECB and the national central banks (NCBs) of the Member States whose currency is the euro. The European System of Central Banks (ESCB) is made up of the ECB and the national central banks of all Member States of the European Union (EU), regardless of whether or not they have adopted the euro. The Governing Board consists of the Executive Committee of the ECB and the governors of individual national banks, and determines the monetary policy, as well as short-term monetary objectives, key interest rates and the extent of monetary reserves.