The EU has treaties which serve as a constitution of sorts – duties, powers, and limits of the EU, and its legal relationship with its member states. These treaties are signed by all member states and together make up the EU’s constitutional basis.

New treaties are signed every now and again with the purpose of amending, extending and redefining previous ones.

There’s e.g. the Maastricht Treaty (1997) which laid the ground work for a single currency and strengthened the power of the European Parliament (each member state has a number of seats and the representatives are elected nationally by a public vote).

The most recent one is the Lisbon Treaty (2009), which among other things, again, shifted the power balance in the EU in favour of the Parliament. It also strengthened EU’s position as a full international legal personality. Other changes were to make the union’s Charter of Fundamental Rights legally binding and to explicitly allow a member state to leave the union.