or abolish almost any written or unwritten element of the constitution. No sitting parliament can pass laws that future parliaments cannot change. The sovereign The United Kingdom is a unitary state under a constitutional monarchy. King Charles III is the monarch and head of state of the UK, as well as 14 other independent countries. These 15 countries are sometimes referred to as "Commonwealth realms". The monarch is formally vested with all executive authority as the personal embodiment of the Crown. The disposition of such powers however, including those belonging to the royal prerogative, is generally exercised only on the advice of ministers of the Crown responsible to Parliament and thence to the electorate. Nevertheless, in the performance of executive duties, the monarch has "the right to be consulted, the right to encourage, and the right to warn". In addition, the monarch has a number of reserve powers at his disposal in order to uphold responsible government and prevent constitutional crises. These reserve powers are particularly relevant to the appointment of a prime minister, preventing unconstitutional use of the British Armed Forces, the prorogation and dissolution of Parliament, the enactment of legislation, and conferring state honours. Parliament The UK is a parliamentary democracy operating under the Westminster system, otherwise known as a "democratic parliamentary monarchy". The Parliament of the United Kingdom is sovereign. It is made up of the House of Commons, the House of Lords and the Crown. The main business of parliament takes place in the two houses, but royal assent is required for a bill to become an act of parliament (law). For general elections (elections to the House of Commons), the UK is divided into 650 constituencies, each of which is represented by a member of Parliament (MP). MPs hold office for up to five years and are always up for re-election in general elections. The Conservative Party, Labour Party and Scottish National Party are, respectively, the current first, second and third largest parties (by number of MPs) in the House of Commons. Prime minister The prime minister is the head of government in the United Kingdom. Nearly all prime ministers have served concurrently as First Lord of the Treasury and all prime ministers have continuously served as First Lord of the Treasury since 1905, Minister for the Civil Service since 1968 and Minister for the Union since 2019. In modern times, the prime minister is,