In monetary union, a “trial marriage” is no longer possible. Reversing that decision would entail incalculable economic and political costs. No one ought to place any faith in that being a realistic option. And thirdly: the euro area is unlike every other currency area, which is defined — at least usually — by the area of a single sovereign nation state.
There is a single monetary policy for the euro area. However, the other areas of policy, such as budget, tax and social policies, are still largely decentralised. Fiscal policy, in particular, must abide by the rules set down in the Maastricht Treaty and in the supplementary Stability and Growth Pact. These three characteristics appear straightforward and self-evident.