Describe the history and consequent deepening as the European Union developed

Which treaties created which level of economic integration? Do countries have the ability to participate in some levels of integration and not others? Give specific examples.

The Treaty of Rome created the basics for a free trade area as well as a political infrastructure for noneconomic integration.
The Single European Act created a common market among the participants and dealt with an outstanding customs union issues.
The Treaty on European Union (or Maastricht Treaty) brought the participating countries into economic union.
Some countries (such as Norway and Switzerland) participate in the common market but are not members of the EU. Only 12 countries currently participate in the treaty on European Union. For example, the UK, Denmark, and Sweden will never be required to use the euro.