This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
No single day marks the founding of the Parliament of the United Kingdom, and that absence of a clean origin story is itself part of what the institution is. Its lineage runs through the Parliament of England, traditionally dated from 1215, and the separate Parliament of Scotland, joined by the Acts of Union 1707 into the Parliament of Great Britain. That body was joined again in 1801 with the Parliament of Ireland to form the Parliament of the United Kingdom of Great Britain and Ireland, the direct ancestor of the institution sitting at the Palace of Westminster today. Three unions, three centuries, and no founding moment any textbook can point to as the single beginning.
What spread from Westminster was not a document but a working method: a bicameral legislature in which a cabinet, drawn from and answerable to an elected House of Commons, holds executive power only so long as it commands that chamber's confidence, with an unelected upper house, the House of Lords, retained as a check rather than a rival source of authority. Because Britain governed roughly a quarter of the globe's population at the height of its empire, that method traveled everywhere British administrators went, and it is why so many former colonies, from India to Canada to Australia to Kenya, built legislatures on a recognizably Westminster pattern even after independence stripped away every other colonial tie. The label mother of parliaments describes that transmission, not a claim that Westminster was the first legislature in history; it was not.
The transmission was never a simple copy. Some successor parliaments kept the fused executive-legislative relationship and discarded the hereditary upper chamber; others federalized power in ways Westminster's own unitary tradition never required; still others took the form while never adopting the unwritten constitutional norms that make the Westminster system function as intended rather than as a set of rules a determined executive can bend. That variation is exactly why political scientists still use Westminster as the reference point for comparison: not because every parliament measured against it should resemble it, but because its particular bargain, cabinet government resting on confidence rather than fixed terms, remains the clearest alternative against which every other design can be read.