The Maastricht Treaty is one of the most fundamental treaties for the European Union, for creating the bloc itself and paving the way for the economic and political union.

The treaty laid foundations for what then became Europe's biggest project for a decade: the European Monetary Union (EMU) and the single currency known as the euro.

It has been two decades since the treaty took effect on November 1, 1993. It was signed by foreign ministers and finances ministers of the then 12 member states in the European Community.

By adding two new areas -- justice and home affairs and a common foreign and security policy -- to the existing European Community, the so-called three pillars of the Union were established.

Within the first pillar, the European Community (EC), the EU's supra-national institutions such as the Commission, the European Parliament and the European Court of Justice had the most power and influence. The other two pillars were essentially more intergovernmental.

The creation of this pillar system led to more cooperation between member states in areas of military, criminal justice, judicial cooperation and foreign policy.

The Maastricht Treaty also set out a number of convergence and stability criteria that had to be met before a country could become a member of the European Monetary Union. Government deficits were limited to be no larger than 3 percent of GDP and inflation was to be no more than 1.5 percentage points above that of the 3 lowest inflation rates in EMU members.

Still, according to some critics, the main deficiency of the treaty was that it removed the power of sovereign states to manage their own economies. The pillar system was eventually abandoned on December 1, 2009 upon the entry into force of the Lisbon Treaty.