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The United Kingdom possesses an unwritten constitution, that is no single constitutional document. Instead Britain's constitution consists of important Acts of Parliament, constitutional conventions and traditions. Many of these govern parliament and the relationship between the parliament, as the legislature, and the executive, Her Majesty's Government.
Parliament fulfils a number of functions.
Parliamentary supremacy arises primarily from the English Civil War in the seventeenth century when Oliver Cromwell leader of the Parliamentary forces defeated the forces of King Charles I and founded the Commonwealth (not to be confused with the modern Commonwealth, which is a club largely though not exclusively made up of ex-UK colonies). Once defeated, in 1649 King Charles I was beheaded (an act known as Regicide) and a form of republican rule instituted over Oliver Cromwell, who ruled as Lord Protector. The Monarchy was reinstated under Charles II in 1660, with Parliament remaining as a full-time part of the system of governance. In the Glorious Revolution of 1688, Parliament deposed the Roman Catholic King James II/VII  and his infant heir and gave the throne to his protestant daughter and her husband, William of Orange. They reigned jointly as Mary II and William III. Parliament excluded all catholic descendants of James II/VII from the throne. Since then the monarch has been generally subservient to parliament. But full parliamentary control of the executive did not occur until the Great Reform Act[?] of 1832, that broadened the voting franchise and so made parliament the full representative of public opinion. Since then, parliament had been dominant, though the monarch still remains an important player in government, with government governing through the Royal Prerogative[?] (ie, powers of the monarch) and with the monarch's formal approval still being required for Acts of Parliament and Orders-in-Council (executive orders).
Since the creation of the Kingdom of Great Britain that unified the crowns of England and Scotland in 1707, parliament has been sovereign (known as the supremacy of parliament), with the power to make or repeal any law. That dominance continued in the United Kingdom of Great Britain and Ireland (formed in 1801 when the crowns of Great Britain and Ireland were merged) and the modern United Kingdom of Great Britain and Northern Ireland (formed in December 1922 when twenty-six of Ireland's thirty-two counties left to form the Irish Free State. However parliament is no longer sovereign. On a national level, the European Union, since Britain's accession to the European Economic Community in 1973, has the power to make laws which must be enforced in each member state, including Britain. On a regional level, a series of sub-parliaments or devolved adminstrations took over the day to day governance of certain regions, specificially Northern Ireland, Scotland and Wales. However, whereas it cannot block european legislation from the European Union (to which at a governmental level it can contribute and help shape) it can vary the powers of the devolved parliaments or even abolish them by a simple Act of Parliament.
A Parliament of sorts (consisting of the Barons, without any commoners) has existed for a very long time; the Barons first started stripping the King's powers away with the Magna Carta in 1215 but Parliament in a more recognisable form did not arise until many centuries later. Also, unlike now, Parliament only convened for short periods of time, and was then disbanded until the King called it again. Kings disliked calling Parliament, because the Members of Parliament were not under their control and so could cause problems. However, Parliament held the purse strings, so the King could not always ignore them, or bribe them, given that parliaments were not representative of public opinion, with many MPs elected from Rotten Boroughs[?] (corrupt constituencies where often only a handful of electors lived and could be bribed to elect a supporter or opponent of the King). These origins are reflected in the modern role of the monarch. It is she who formally summons, prorogues and dissolves parliament. At the moment that the latter royal order is made, all the MPs cease to hold office, with the resultant vacancies being filled by a general election. However this is always done on the advice (instruction) of Her Majesty's Government, which as it is answerable to parliament does parliament's wishes, or at least does not ignore the view of parliament, though parliament itself is not consulted.
 James II/VII's two ordinals denote his position as King James II of England and King James VII of Scotland. After England and Scotland were joined by the 1707 Act of Union British monarchs ceased to use Scottish ordinals. Were they still doing so, the modern Queen Elizabeth II would be Queen Elizabeth II/I, reflecting the fact that there never was a Queen Elizabeth I of Scotland. In fact GPO letter boxes in Scotland do not have EIIR for the insignia for this very reason. They use ER for the insignia unlike English, Welsh and Northern Irish letter boxes.