Sao Tome and Principe is an island nation in the Gulf of Guinea off the western coast of Central Africa. The two main islands are mountainous and form part of a chain of extinct volcanoes. The government system is a republic; the chief of state is the president, and the head of government is the prime minister. Sao Tome and Principe has a mixed economic system in which there is a variety of private freedom, combined with centralized economic planning and government regulation. Sao Tome and Principe is a member of the Economic Community of Central African States (ECCAS).