Le texte en entier
The European treaty is silent about how to write off the euro. Nonetheless, the Vienna Convention provide that States have the right to withdraw from a treaty.
In addition, article 50 of the Treaty of Lisbon provides that Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements. Nevertheless, the treaty is silent about the eurozone.
There are a binary choices, either we leave unilaterally the eurozone, or we organize a coordinate exit of the eurozone.
First of all, a State can leave unilaterally the eurozone.
On the one hand, a State can leave the European Union because the article 50 of the Treaty of Lisbon provides that Any Member State may decide to withdraw from the Union. If it leaves the Union, it would automatically write off the euro.
On the other hand, a State can leave unilaterally the eurozone and stay in the Union. The treaty allows to be membership of the Union and to keep its former national currency.
For example, the United Kingdom and the Danemark are in the Union but not in the eurozone.
However there is a problem. An exit from the euro would weaken it. Maybe the currency will not survive it. If the eurozone collapses, it would be a financial crisis for all member States of the Union.
That is why it will first warn the partners to be able to organize a joint and coordinated exit.
Secondly, a State can agree with the Union to leave the eurozone. A coordinated withdraw from the euro can allow, in full knowledge, to prepare the futur. It would take time.
First, States that leave the eurozone would return to their former currency. For example the franc for France.
Moreover, the new national currency will decree as equivalent with the former euro, to avoid a financial collapse. Therefore the prices won't change for the population.
Besides, the value of the new currency will be adjusted according to the economic strength of the state. The low powered state's currency will be devalued. In others word, it will lose more value than the euro
To conclude, withdraw the euro will be a first crack in the Union. A gap that threaten the Union. Other country might follow and write off the euro. It will create a two-tier Union. Nevertheless, it is not very probable.