The Treaty of Lisbon was signed on 13 December 2007 between 27 European states that are members of the European Union (EU). It became effective on December 1, 2009. It is now the document that defines the Union, but it is not a constitution. It gives a common set of rules that the member states have agreed to use on subjects where they have decided to work together. It does better than previous treaties such as the Treaty of Rome and the Treaty of Maastricht. It gives the European Union new things, such as: