Greece can become the next Montenegro

The day after a formal default, Greek banks would no longer have access to the regular monetary policy operations of the European Central Bank because the ECB could no longer accept their collateral—Greek debt—which would immediately have junk status. The country would thus effectively cease to be part of the euro area. Its status would resemble that of Montenegro, which adopted the euro as legal tender without officially being a member of the single currency zone.

In Greece, following a messy default, euro notes and coins would still circulate in the economy, but one euro in a Greek bank account would no longer be automatically equivalent to a euro in a bank account elsewhere in the euro area, as Greek banks might immediately become insolvent and thus be shut out of the payment systems. Until Greek solvency was re-established, the euro zone would thus de facto have lost one of its members, even though the Greek Central Bank head would still sit on the Governing Council of the ECB and the Greek finance minister would still be a member of the Euro Group, with their normal voting powers intact.