Canada is an independent constitutional monarchy and a member of the Commonwealth of Nations. The monarch of Great Britain and Northern Ireland is also the monarch of Canada and is represented in the country by the office of governor-general. The basic constitutional document is the Canada Act of 1982, which replaced the British North America Act of 1867 and gave Canada the right to amend its own constitution. The Canada Act, passed by Great Britain, made possible the Constitution Act, 1982, which was passed in Canada. The document includes a Charter of Rights and Freedoms, which guarantees the rights of women and native peoples and protects other civil liberties.
Power on the federal level is exercised by the Canadian Parliament and the cabinet of ministers, headed by the prime minister. (See the table entitled Canadian Prime Ministers since Confederation for a list of Canada's prime ministers.) The federal government has authority in all matters not specifically reserved to the provincial governments, and may veto any provincial law. The provincial governments have power in the fields of property, civil rights, education, and local government; they may levy only direct taxes. Canada has an independent judiciary; the highest court is the Supreme Court, with nine members.
The Parliament has two houses: the Senate and the House of Commons. There are generally 105 senators, apportioned among the provinces and appointed by the governor-general upon the advice of the prime minister. Senators may serve until age 75; prior to 1965 they served for life. The 308 members of the House of Commons are elected, largely from single-member constituencies. Elections must be held every four years, but the Commons may be dissolved and new elections held earlier if the government loses a confidence vote. There are four main political parties: the Liberal party, the Conservative party (formed in 2003 by the merger of the Canadian Alliance and the Progressive Conservative party), the Bloc Québécois (aligned with the Parti Québécois of Quebec), and the New Democratic party.