Belgium is a country in Western Europe that borders the North Sea. Neighboring countries include France, Germany, Luxembourg, and the Netherlands. Flat coastal plains take up most of the northwest, but rugged mountains of the Ardennes Forest lie in the southeast. The government system is a federal parliamentary democracy under a constitutional monarchy. The chief of state is the king, and the head of government is the prime minister. Belgium has a mixed economic system which includes a private-enterprise system, combined with centralized economic planning, and government regulation. Belgium is a member of the European Union (EU).