The House of Commons plays the major role in law-making. It consists of Members of Parliament (called MPs for short), each of whom represents an area in England, Scotland, Wales or Northern Ireland. MPs are elected either at a general election, or at a by-election following the death or retirement of an MP. Parliamentary elections must be held every five years, but the Prime Minister can decide on the exact date within those five years. The minimum voting age is 18, and the voting is taken by secret ballot. The election campaign lasts about three weeks. The election is decided on a simple majority— the candidate with most votes wins. An MP who wins by a small number of votes may have more votes against him (that is, for the other candidates) than for him