The Treaty says that, before it can join, a country must have brought its economy into line with the euro area – and that this convergence must not only be observed at the moment of entry, but also be sustainable in the medium and long term.
But history shows that the euro area is not a “closed shop”: we started in 1999 with 11 countries. When Slovakia joins on 1 January 2009 there will be 16 euro area countries. The euro area will then have around 329 million citizens.