The English Treasury seems to have come into existence around 1126, in the reign of Henry I. The treasury emerged out of the royal household, and served as the location where the king kept his treasures. The head of the treasury was called the Lord Treasurer or Lord High Treasurer. Starting in Tudor times, the Lord Treasurer became one of the Great Officers of State, and ranked third, immediately beneath the Lord Chancellor and above the Lord President of the Council.
Beginning in the 17th Century, the Treasury was frequently entrusted to a commission, rather than to a single individual, and after 1714, it was always in commission. The commissioners were referred to as Lords Commissioners of the Treasury, and given a number based on seniority. Eventually, the First Lord of the Treasury came to be seen as the natural head of any ministry, and from Robert Walpole on, began to be known, unofficially, as the prime minister. Before 1827, the First Lord of the Treasury, when a commoner, also held the office of Chancellor of the Exchequer, while if the First Lord was a peer, the 2nd Lord would usually serve as Chancellor. Since 1827, the Chancellor of the Exchequer has always been Second Lord of the Treasury when he has not also been the Prime Minister. By convention, the other Lords Commissioners of the Treasury are the Government's Whips in the House of Commons.
Contrary to popular impression, 10 Downing Street is the residence of the First Lord of the Treasury, not the prime minister. There is in fact no prime ministerial residence, though as all modern prime ministers have simultaneously been first Lord of the Treasury, 10 Downing Street has come to be closely identified with the premiership. Similarly, 11 Downing Street is the residence of the Second Lord of the Treasury, not the residence of the Chancellor of the Exchequer, though given that since 1827 all chancellors have also been second lord, people often presume wrongly that 11 Downing Street is the Chancellor's residence.
See also
List of Commissioners of the Treasury, Secretary to the Treasury, UK topics
Sources
Origins
The Lords of the Treasury
Official Residences
List of First Lords of the Treasury, 1714-1905
for earlier Lord Treasurers and First Lords, see List of Lord Treasurers. Much of this list overlaps with the list of Prime Ministers of the United Kingdom, but there are some notable differences.
thereafter the First Lord of the Treasury has always been identical to the Prime Minister of the United Kingdom