North Rhine-Westphalia is the most populous state of Germany, with a population of approximately 18 million, and the fourth largest by area. Its capital is Düsseldorf; the most populous city is Cologne. Four of Germanys ten largest cities—Cologne, Düsseldorf, Dortmund, and Essen—are located within the state, as well as the largest metropolitan area on the European continent, Rhine-Ruhr. North Rhine-Westphalia was formed in 1946 as a merger of the provinces of North Rhine and Westphalia, both formerly parts of Prussia, and the Free State of Lippe. The state has been run by a coalition of the Social Democrats and Greens since 2010. It makes up almost a quarter of the population and a quarter of the economy of Germany.