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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:

• Informal – unilateral adoption of a foreign currency.^[1]
• Formal – adoption of foreign currency by virtue of bilateral or multilateral agreement with the monetary authority, sometimes supplemented by issue of local currency in currency peg regime.
• Formal with common policy – establishment by multiple countries of a common monetary policy and monetary authority for their common currency.

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.^[2]

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.

• A currency union helps its members strengthen their competitiveness on a global scale and eliminate the exchange rate risk.
• Transactions among member states can be processed faster and their costs decrease since fees to banks are lower.^[3]
• Prices are more transparent and so are easier to compare, which enables fair competition.
• The probability of a monetary crisis is lower. The more countries there are in the currency union, the more they are resistant to crisis.

• The member states lose their sovereignty in monetary policy decisions. There is usually an institution (such as a central bank) that takes care of the monetary policymaking in the whole currency union.
• The risk of asymmetric "shocks" may occur. The criteria set by the currency union are never perfect, so a group of countries might be substantially worse off while the others are booming.
• Implementing a new currency causes high financial costs. Businesses and also single persons have to adapt to the new currency in their country, which includes costs for the businesses to prepare their management, employees, and they also need to inform their clients and process plenty of new data.
• Unlimited capital movement may cause moving most resources to the more productive regions at the expense of the less productive regions. The more productive regions tend to attract more capital in goods and services, which might avoid the less productive regions.^[4][5]

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.^[6]

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.^[7]

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.^[8][9]

Currency	Union	Users	Est.	Status	Population
CFA franc	Issued by the (French) Overseas Issuing Institute between 1945 and 1962 then by the Central Bank of West African States and the Bank of Central African States	West African CFA franc users:  Benin  Burkina Faso  Côte d'Ivoire  Guinea-Bissau  Mali  Niger  Senegal  Togo Central African CFA franc users:  Cameroon  Central African Republic  Chad  Equatorial Guinea  Gabon  Republic of the Congo	1945	Formal, common policy	151,978,440
CFP franc	Issued by the (French) Overseas Issuing Institute	French Polynesia  New Caledonia  Wallis and Futuna	1945	Formal, common policy	552,537
Eastern Caribbean dollar	Eastern Caribbean Currency Union of the Eastern Caribbean Central Bank (ECCB) and the OECS.	Anguilla  Antigua and Barbuda  Dominica  Grenada  Montserrat  Saint Kitts and Nevis  Saint Lucia  Saint Vincent and the Grenadines	1965	Formal, common policy de facto EMU for CSME members[10]	625,000
Euro	International status and usage of the euro	Eurozone:  Austria  Belgium  Croatia  Cyprus  Estonia  Finland  France  Germany  Greece  Ireland  Italy  Latvia  Lithuania  Luxembourg  Malta  Netherlands  Portugal  Slovakia  Slovenia  Spain and EU special territories:  French Southern and Antarctic Lands  Saint Barthélemy  Saint Pierre and Miquelon  Akrotiri and Dhekelia  Andorra  Kosovo  Monaco  Montenegro  San Marino  Vatican City	1999/2002	Formal, common policy and EMU for EU members Formal for Monaco and Akrotiri and Dhekelia (which form part of the EU's customs territory) Informal for Kosovo, Montenegro Formal for Andorra and San Marino (which are in customs union with the EU's customs territory)	341,008,867
Singapore dollar Brunei dollar	Managed together by the Monetary Authority of Singapore	Brunei  Singapore	1967	Formal; currencies mutually exchangeable[11]	5,137,000
Australian dollar		Australia and external territories:  Ashmore and Cartier Islands  Australian Antarctic Territory  Christmas Island  Cocos (Keeling) Islands  Coral Sea Islands Territory  Heard Island and McDonald Islands  Norfolk Island  Kiribati  Nauru  Tuvalu	1966	Informal	24,557,000
Pound sterling	Sterling area (former)	United Kingdom and Overseas Territories:  British Antarctic Territory  British Indian Ocean Territory  Falkland Islands  Gibraltar  Saint Helena, Ascension and Tristan da Cunha  South Georgia and the South Sandwich Islands and Crown Dependencies:  Bailiwick of Guernsey  Bailiwick of Jersey  Isle of Man	1939	Semi-formal. UK banknotes are legal tender in locations outside the UK. Local currencies are pegged to the GBP but not necessarily accepted in the UK: Guernsey pound, Manx pound, Jersey pound and Alderney pound, Falkland Islands pound, Gibraltar pound, Saint Helena pound	62,321,000
Indian rupee		India  Bhutan[12]    Nepal[13]	1974	Informal Nepal minor usage	1,352,000,000
New Zealand dollar		New Zealand and Realm:  Cook Islands  Niue  Tokelau  Pitcairn Islands	1967	Informal	4,411,000
Israeli new sheqel		Israel  Palestine	1927/1986	Informal	11,738,000
Jordanian dinar[14][15]		Jordan  Palestine (West Bank only)		Informal	8,922,000
Russian ruble		Russia  Abkhazia  South Ossetia	2008	Informal	142,177,000
South African rand	Multilateral Monetary Area	Lesotho  Namibia  South Africa  Eswatini	1974	Formal de facto customs and monetary union for the SACU member countries	52,924,669
Swiss franc		Liechtenstein   Switzerland	1920	Informal de facto economic and monetary union—1924 creation of a customs union, then members of the European Free Trade Association (a common market), and now also part of the European Single Market.	8,547,015
Turkish lira		Turkey  Turkish Republic of Northern Cyprus	1983	Informal	75,081,100
United States dollar		United States and insular areas:  American Samoa  Guam  United States Minor Outlying Islands  Northern Mariana Islands  Puerto Rico  United States Virgin Islands and Compact of Free Association members:  Marshall Islands  Federated States of Micronesia  Palau  Ecuador  El Salvador  Panama  Timor-Leste  Turks and Caicos Islands  British Virgin Islands BES islands	1904 (Panama only)	Formal for insular areas and sovereign status with Compact of Free Association,[16] informal for other areas	339,300,000

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.^[17]

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.^[18]

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.^[19]

Community	Currency	Region	Target date	Notes
East African Community	East African shilling	Africa	2012 (not met), 2015 (not met), 2024[20]
West African Monetary Zone	Eco	Africa	2027	Inside Economic Community of West African States, planned to eventually merge with West African franc
ASEAN+3	Asian Monetary Unit [citation needed]	Asia	?	a free trade agreements matrix partially established
Cooperation Council for the Arab States of the Gulf	Khaleeji	Arabian Peninsula	?	Oman and the United Arab Emirates do not intend to adopt the currency at first but will do at a later date.
African Economic Community	Afro or Afriq	Africa	2028[21]	Planned for 2028 or later
Brazil, Argentina and possibly other countries	Sur	Latin America	?	As Financial Times reports, Brazil and Argentina will announce in January 2023 that they are starting preparatory work on a common currency "Sur" (South). The initiative would later be extended to invite other Latin American nations.[22]

• between Bahrain and Abu Dhabi using the Bahraini dinar
• between Bahrain, Kuwait, Oman, Qatar and the Trucial States, using the Gulf rupee from 1959 until 1966
• between Aden,  South Arabia, Bahrain, Kenya, Kuwait, Oman, Qatar, British Somaliland, the Trucial States, Uganda, Zanzibar and British India (later independent India) using the Indian rupee until 1974
• between Belgium and the Grand-Duchy of Luxemburg (Belgium-Luxembourg Economic Union) using the Belgian/Luxembourgish franc from 1921 to the Euro
• between British India and the Straits Settlements (1837–1867) using the Indian rupee
• between Czech Republic and Slovakia (briefly from January 1, 1993 to February 8, 1993) using the Czechoslovak koruna
• between Ethiopia and Eritrea using the Ethiopian birr
• between France, Monaco, and Andorra using the French franc
• between Austria-Hungary and Liechtenstein using the Austro-Hungarian krone
• between the Eastern Caribbean, Jamaica, Barbados, Trinidad and Tobago and British Guiana using the British West Indies dollar
• between the Eastern Caribbean, Barbados, Trinidad and Tobago and British Guiana using the Eastern Caribbean dollar
• between Italy, Vatican City, and San Marino using the Italian lira
• between Jamaica and the Cayman Islands using the Jamaican pound and later Jamaican dollar
• between Kenya, Uganda, and Zanzibar using the East African rupee
• between Kenya, Uganda, and Zanzibar (and later  Tanganyika) using the East African florin
• between Kenya,  Tanganyika and Zanzibar (later merged as Tanzania), Uganda, South Arabia, British Somaliland and Italian Somaliland using the East African shilling
• Latin Monetary Union (1865–1927), initially between France, Belgium, Italy and Switzerland, and later involving Greece,^[23] Romania,  Spain and other countries.
• between Liberia and the United States using the United States dollar
• between Mauritius and Seychelles using the Mauritian rupee
• between Nigeria, the Gambia, Sierra Leone, the Gold Coast and Liberia using the British West African pound
• between Prussia and the North German states (1838–1857) using the North German thaler
• between Russia and the former Soviet republics (1991–1993) using the Soviet ruble
• between Qatar and all the emirates of the United Arab Emirates, except Abu Dhabi using the Qatari and Dubai riyal
• between Saudi Arabia and Qatar using the Saudi riyal
• between Western Samoa and New Zealand using the New Zealand pound
• Scandinavian Monetary Union (1870s until 1924), between Denmark, Norway and Sweden^[23]
• between the Solomon Islands, Papua New Guinea and Australia using the Australian dollar
• between Australia, Papua, New Guinea, Nauru, the Solomon Islands, and the Gilbert and Ellice Islands using the Australian pound
• between Bavaria, Baden, Württemberg, Frankfurt, and Hohenzollern using the South German guilder
• between Spain and Andorra using the Spanish peseta
• between Trinidad and Tobago and Grenada using the Trinidad and Tobago dollar
• between Brunei, Malaysia, and Singapore (1953–1967) using the Malaya and British Borneo dollar
• between Cambodia, Laos, Guangzhouwan, Annam, Tonkin, and Cochinchina (later Vietnam) between 1885 and 1952 using the French Indochinese piastre
• between  South Africa, South West Africa, and Bechuanaland (later independent Botswana) using the South African rand
• between Egypt, Anglo-Egyptian Sudan, and Mandatory Palestine (until 1926) using the Egyptian pound
• between West Germany and East Germany between 1 July 1990 and 3 October 1990, as part of a temporary, so-called "Monetary, Economic and Social Union" prior to German reunification.
• between what ultimately became the Republic of Ireland and the United Kingdom, between 1928 and 1979. The Irish Pound was held at exactly the same value as Sterling for this period, although it was not accepted for payments in the UK.
• Yen Bloc (between 1905 and 1945), between the Empire of Japan, the Korean Empire, Manchukuo, Mengjiang, the Wang Jingwei regime, and Japanese-occupied Southeast Asia prior to and during World War II.

• proposed Pan-American monetary union – abandoned in the form proposed by Argentina
• proposed monetary union between the United Kingdom and Norway using the pound sterling during the late 1940s and early 1950s
• proposed gold-backed, pan-African monetary union put forward by Muammar Gaddafi prior to his death

• List of pegged currencies
• North American Currency Union (Amero)^[24]

• Acocella, N. and Di Bartolomeo, G. and Tirelli, P. [2007], ‘Monetary conservatism and fiscal coordination in a monetary union’, in: ‘Economics Letters’, 94(1): 56–63.
• Bergin, Paul (2008). "Monetary Union". In David R. Henderson (ed.). Concise Encyclopedia of Economics (2nd ed.). Indianapolis: Library of Economics and Liberty. ISBN 978-0865976658. OCLC 237794267.

• West Africa opts for currency union
• Economist- Antipodean currencies (Australia and New Zealand)
• Reasons for the collapse of the Rouble Zone
• OECD Development Centre – the Rand Zone