The UK Parliament consists of three elements:
One Member of Parliament is elected for each of the 650 constituencies across the country. Under what has been called the First Past the Post System, the candidate who receives most votes is elected.
General elections have to take place no longer than every 5 years. The Fixed Terms Parliament Act, 2011 now sets the date of the general election as the first Thursday in May. This is five years after the last general election, though there are mechanisms to hold it earlier if the Government becomes unstable or exceptional circumstances. This happened in 2017. Theresa May called a General Election, with parliamentary approval, following the decision to leave the European Union in 2016 and David Cameron’s resignation.
MPs who die or decide to leave the House of Commons are replaced within a month or more at a by-election.
The Commons agrees laws and decides taxation and spending and can vote to remove the Government.
The House of Lords consists of around 800 unelected members almost all of whom remain there for life. New members are appointed by the Prime Minister.
The Lords can veto or amend laws passed by the Commons but since the Parliament Act, 1911 the Commons has been able to overrule them. It cannot interfere with taxation or spending.
There is the legal idea of the Monarch (King or Queen) in Parliament.
Only the Monarch has the power to call and dissolve Parliament.
The Monarch has to approve all legislation once it has been agreed by the Lords and Commons, though the last monarch to refuse to do so was Queen Anne in 1708.