The Commons is made up of 646 elected members of Parliament, known as MPs. They are elected by the first-past-the-post system, which is also known as the plural majority system. Elections must be held at least every five years but the actual date is set by the sitting Prime Minister who asks the monarch to dissolve Parliament. Parliaments normally last about four years.
Anyone over 21 who is a British, Irish or Commonwealth citizen can stand except peers, bishops, undischarged bankrupts, prisoners and certain officeholders including judges, services personnel, police officers and other civil servants (persons holding offices of profit under the Crown). People convicted of certain corruption offences and members of non-Commonwealth legislatures may not stand. It is widely held that 'idiots' and 'lunatics' may not stand.
There are certain rules governing the timetable for elections. The Session of Parliament is prorogued on the day scheduled and dissolution normally follows on the same day, although it does not have to. It is normally accompanied by the proclamation calling a new Parliament. The election must normally take place 17 days after this proclamation (not including weekends and public or bank holidays). Election campaigns - the time between dissolution and the election - normally last about three or four weeks.
For the general election, the United Kingdom is split into 646 geographical single-member constituencies. By area, the largest constituency is 918,319 hectares and the smallest is 727 hectares. Anyone who is over 18 and on the electoral roll can vote. Each voter casts one vote for an individual candidate of a registered political party or for an independent. The candidate with the most votes, though not necessarily the majority of votes, is returned as the MP for that constituency.
If an MP resigns or dies, then a by-election is held. The date for the by-election is chosen by the Government but it is announced by the Speaker.
The word 'Parliament' is used to refer to the time between two general elections. It is made up of Sessions, which normally last about a year from November to October. A Session runs from the State Opening of Parliament (the Queen's Speech) until prorogation or dissolution, so there are normally four Sessions during each Parliament. After an election, the Queen opens Parliament in the normal way. The first Session after an election is normally longer than a year, running to the following October.