The Grand Duchy of Luxembourg is a constitutional hereditary monarchy. The constitution, proclaimed in 1868 and later amended (particularly in 1919), provides for a democratic government, with legislative power unconditional in a unicameral chamber of deputies composed of 60 members elected at least every five years by universal vote of persons aged 18 and older. The sovereign (the grand duke or grand duchess) has the constitutional right to organize the government, which consists of a prime minister, who is head of the government, and at least three other ministers. A council of state, appointed for life by the sovereign, acts in an advisory capacity. The three major political parties are the Christian Social Party, the Socialist Worker Party, and the Democratic Party. The nation is separated into 12 cantons, each of which is subseparated into communes. A member of the North Atlantic Treaty Organization (NATO), Luxembourg maintains a small volunteer army consisting of about 899 members.