Following the Smithsonian Agreement, member states of the European Economic Community adopted a narrower currency band of 1.125% for exchange rates among their own currencies, creating a smaller scale fixed exchange rate system known as the "snake in the tunnel". The snake proved unsustainable as it did not compel EEC countries to coordinate macroeconomic policies. In 1979, the European Monetary System (EMS) phased out the currency snake. The EMS featured two key components: the European Currency Unit (ECU), an artificial weighted average market basket of European Union members' currencies, and the Exchange Rate Mechanism (ERM), a procedure for managing exchange rate fluctuations in keeping with a calculated parity grid of currencies' par values. The parity grid was derived from parities each participating country established for its currency with all other currencies in the system, denominated in terms of ECUs. The weights within the ECU changed in response to variances in the values of each currency in its basket. Under the ERM, if an exchange rate reached its upper or lower limit (within a 2.25% band), both nations in that currency pair were obligated to intervene collectively in the foreign exchange market and buy or sell the under- or overvalued currency as necessary to return the exchange rate to its par value according to the parity matrix. The requirement of cooperative market intervention marked a key difference from the Bretton Woods system. Similarly to Bretton Woods however, EMS members could impose capital controls and other monetary policy shifts on countries responsible for exchange rates approaching their bounds, as identified by a divergence indicator which measured deviations from the ECU's value. The central exchange rates of the parity grid could be adjusted in exceptional circumstances, and were modified every eight months on average during the systems' initial four years of operation. During its twenty-year lifespan, these central rates were adjusted over 50 times.