Greece: What Could Happen If Country Leaves the European Union

— -- If Greece and its 11 million people left the European Union, it would be an unprecedented move within the monetary system that gave birth to the currency known as the euro.

A referendum, which has already been criticized as confusing and illegal, scheduled on Sunday will help determine whether a "Grexit" -- Greek exit -- from the 19-nation currency bloc could become a reality. Earlier this week, the government issued a maximum cash withdrawal for Greek residents at 60 euros. But due to the shortage of 20-euro banknotes, people mostly have access to 50-euro banknotes.

In one branch of the National Bank of Greece in Athens, the counters were open only for pensioners who do not have credit cards. By order of the government, these seniors can withdraw only 120 euros per week. In line, one senior explains how they are facing the latest cash shortage and instability with dignity.

"You know, I survived the German occupation, where we had nothing to eat; the civil war between communists and royalists, where we had no national harmony; the dictatorship of the colonels, where we had more freedom; the current crisis for me is not much," said Angela Pisikouri, 79, former official at the Ministry of Economy. "I will vote no in the referendum on Sunday, because I think that we, Greeks, must have the right to say what we think."

Here are some of the things that could happen if Greece ultimately leaves the European Union:

1. Nationalize Greece's Banking System

One source with Greece's ruling Syriza party said leaders have discussed the possibility of nationalizing the country's banking system, according to The Telgraph.

"We will shut down the banks and nationalise them, and then issue IOUs if we have to, and we all know what this means. What we will not do is become a protectorate of the E.U.,” the source told The Telegraph.

2. Introduce a New Currency

A country has never left the E.U., so it's difficult to predict what Greece would do once the euro loses its legal tender status there, experts said.

A parallel currency could be used for a while, Hari Tsoukas, a professor of organization studies at Warwick Business School in the U.K., told ABC News.

Greece became a member of the E.U. in 2001 and adopted euro banknotes and coins in January 2002 after a transitional period. The dual circulation period, "when both the Greek drachma and the euro had legal tender status," ended on Feb. 28, 2002, according to the EU's website.

3. Return to the Drachma

Tsoukas said it's likely that Greece would eventually revert to the drachma if the euro is no longer its main currency.

If the Greeks reverted to the drachma, the value of the Greek currency could drop considerably, with depreciation in the level of 40 percent against the dollar due to a large sell off of the currency, Valentin Marinov, head of Group-of-10 currency research at Credit Agricole SA in London, told Bloomberg.