Monetary union

A currency union (also known as monetary union) involves two or more states sharing the same currency without them necessarily having any further integration (such as an economic and monetary union, which would have, in addition, a customs union and a single market).

Three types of currency unions exist:

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.

 Austria
 Belgium
 Cyprus
 Estonia
 Finland
 France
 Germany
 Greece
 Ireland
 Italy
 Latvia
 Lithuania
 Luxembourg
 Malta
 Netherlands
 Portugal
 Slovakia
 Slovenia
 Spain

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Wikipedia