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. England has existed as a unified entity since the 10th century; the union between England and Wales, begun in 1284 with the Statute of Rhuddlan, was not formalized until 1536 with an Act of Union; in another Act of Union in 1707, England and Scotland agreed to permanently join as Great Britain ; the legislative union of Great Britain and Ireland was implemented in 1801, with the adoption of the name the United Kingdom of Great Britain and Ireland; the Anglo-Irish treaty of 1921 formalized a partition of Ireland; six northern Irish counties remained part of the United Kingdom as Northern Ireland and the current name of the country, the United Kingdom of Great Britain and Northern Ireland, was adopted in 1927.