What is the Westminster System?
The Westminster System is a method of parliamentary government, also known as responsible government, which evolved in England and was adopted in its colonies (now forming most of the countries within the Commonwealth), including Australia and which then was adopted in varying formats in several non-British countries.
It is based on the principle that the executive government is responsible to the people through the parliament. The executive government including the Prime Minister, is formed by those who command the support of the lower House of Parliament. Ministers must be members of a House of Parliament and are accountable to it. The original and proper Westminster system is established under the Crown which means that the monarch is head of state but with limited powers. The (proper) system also comprises an independent public service and an independent judiciary that applies the rule of law.
The Westminster System developed over several centuries of tussle between the English parliament and its king which included a civil war in the 17th century and the establishment of a short-lived republic. In the 13th century a charter was granted to the people called Magna Carta which gave taxing powers to what was to eventually become the parliament thus giving it authority it never possessed before and leading to its supremacy.
The re-establishment of the monarchy was on agreed terms which meant a greater independence for the parliament which by the latter 19th century had evolved pretty much into the system we know today.