The convergence criteria laid down in the Maastricht Treaty are the yardstick for this. These criteria set unambiguously verifiable requirements in terms of price stability, long-term interest rate levels, the government fiscal position, and participation in the exchange-rate mechanism of the European Monetary System, ERM II.
In addition, pursuant to Article 121 of the EU Treaty, the examination of the sustainability of achieved convergence “should also take account of ... the results of the integration of markets, the situation and development of the balances of payment on current account and ... the development of unit labour costs and other price indices”.