At the start of the Twentieth Century, the Prime Minister, Lord Salisbury, sat in the House of Lords. By 1963 it was an accepted convention that Prime Ministers could only sit in the House of Commons. No law was ever passed; no resolution tabled in the House of Commons - the convention arose because the constitutional actors believed that it bound them.
Clearly the change in powers in the Parliament Act of 1911 altered the relative importance of each House - but there was no express suggestion in that statute about where the Prime Minister should sit. The key milestones were:-
1900 – Lord Salisbury is Prime Minister (until 1902)
1923 – Lord Curzon is considered as possible successor to Bonar Law, but Stanley Baldwin is chosen. The view was prevalent that this was because Curzon was a Peer.
1940 – Lord Halifax, though more popular within the Conservative Party, does not take over from Chamberlain, as doubts expressed about whether a Peer should be Prime Minister
1963 – Lord Home stands down as a Peer and is elected to House of Commons after becoming Prime Minister. Lord Hailsham had renounced his peerage in the hope of becoming Prime Minister.
