The United Kingdom is a constitutional monarchy and parliamentary democracy, with a queen and a Parliament that has two houses: the House of Lords, with 574 life peers, 92 hereditary peers, and 26 bishops; and the House of Commons, which has 651 popularly elected members. Supreme legislative power is vested in Parliament, which sits for five years unless dissolved sooner. The House of Lords was stripped of most of its power in 1911, and now its main function is to revise legislation. In Nov. 1999 hundreds of hereditary peers were expelled in an effort to make the body more democratic. The executive power of the Crown is exercised by the cabinet, headed by the prime minister.