The Prime Minister, along with the other ministers of the Cabinet, is formally appointed by the Governor General on behalf of the Queen. However, by constitutional convention designed to maintain stability in government, the Governor General will almost always call on the leader of the party which holds the most seats in the House of Commons to form a government.
The Prime Minister may be any Canadian Citizen of voting age (18 years). It is customary for the Prime Minister to also be a sitting member of the House of Commons, although two Prime Ministers have governed from the Senate: Sir John Joseph Caldwell Abbott and Sir Mackenzie Bowell. (Both men, in their roles as Government Leader in the Senate, succeeded Prime Ministers who died in office in the 1890s; Canadian convention has since evolved toward the appointment of an interim leader in such a scenario.) One Prime Minister, William Lyon Mackenzie King, having lost his own seat in a general election while his party retained a plurality in the House of Commons, briefly governed from the hallway, until he won a by-election a few weeks later.
If the prime minister should fail to win his or her seat, a junior Member of Parliament in a safe seat would typically resign to permit a by-election to elect that leader to a seat. However, if the leader of the governing party is changed shortly before an election is due and the new leader is not a Member of Parliament, he or she will normally await the general election before running for a seat. For example, John Turner was briefly prime minister in 1984 without being a member of the House of Commons; he would ironically win his seat in the general election that swept his party from power. The official residence of the prime minister is 24 Sussex Drive in Ottawa, Ontario. All prime ministers (with the exception of Kim Campbell) have lived there since Louis St. Laurent in 1951. The prime minister also has a secondary residence at Harrington Lake in Gatineau Park near Ottawa.
In earlier years, it was tradition that the Sovereign bestow a knighthood on each new Canadian prime minister. As such, several carry the prefix "Sir" before their name (of the first eight prime ministers, only Alexander Mackenzie refused knighthood). After the Nickle Resolution debate of 1919, it was against policy for the Sovereign to grant titles to Canadians; the last prime minister knighted was Sir Robert Laird Borden, who was in power when the Nickle Resolution was debated. [It was never actually passed into effect.] In addition one prime minister, Richard Bennett, was created a viscount after his retirement and the widow of Sir John A. Macdonald was created a baroness.