Concentric circles
The idea that the EU can be organised in different circles with states at the core being members of all areas of integration and those at the periphery not.
France has proposed a full formal federation for the core countries in the EU, especially those using the euro, inside a broader confederation for all. This is provided for in the Lisbon Treaty through the concepts of enhanced cooperation and structured cooperation for defence.
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See also Variable geometry, a term used to describe a situation where EU members can move between different co-operation or integration areas.