Great Britain is a constitutional monarchy. The constitution exists in no one document but is a centuries-old accumulation of statutes, judicial decisions, usage, and tradition. The hereditary monarch, who must belong to the Church of England according to the Act of Settlement of 1701, is almost entirely limited to exercising ceremonial functions.
Sovereignty rests in Parliament, which consists of the House of Commons, the House of Lords, and the crown. Effective power resides in the Commons, whose 659 members are elected from single-member constituencies. The executive—the cabinet of ministers headed by the prime minister—is usually drawn from the party holding the most seats in the Commons; the monarch usually asks the leader of the majority party to be prime minister. Elections must be held at least once in five years, but within that period the prime minister may at any time request the crown to dissolve Parliament and call for new elections. Most legislation originates in the Commons. Traditionally, the hereditary and life peers of the realm, high officials of the Church of England, and the lords of appeal (who exercise judicial functions) had the right to sit in the House of Lords. In 1999 both houses voted to strip most hereditary peers of their right to sit and vote in the chamber. The House of Lords may take a part in shaping legislation, but it cannot permanently block a bill passed by the Commons, and it has no authority over money bills. The lords of appeal constitute the highest court in Great Britain. The crown need not assent to all legislation, but assent has not been withheld since 1707.
The two main parties are the Conservative party, descended from the old Tory party, and the Labour party, which was organized in 1906 and is moderately socialist. The Liberal Democrats, formed by a merger of the Liberal party and the Social Democratic party, is a weaker third party. Both Scotland and Wales have nationalist parties whose goal is the independence of those respective regions.