Danish opt-outs from the European Union
Denmark holds opt-outs from European Union policies in relation to police and justice and the adoption of the euro. They were secured under the Edinburgh Agreement in 1992 after a referendum for the ratification of the Maastricht Treaty was rejected by Danish voters, as a package of measures to assuage concerns raised during that referendum.
Source: Wikipedia — Danish opt-outs from the European Union (CC BY-SA 4.0)