Tonga is governed under the constitution of 1875 as revised. The monarch is the head of state, and the government is headed by the prime minister, who is elected by the Legislative Assembly. The unicameral Legislative Assembly has 26 seats, 9 for nobles and 17 for representatives elected by popular vote; all serve four-year terms. Tonga is a member of the Commonwealth of Nations. Administratively, the country is divided into the three island groups.