London , England ( CNN ) -- The United Kingdom votes on a new government May 6 through a political system that dates back centuries . When does an election happen ? The UK has no written constitution . Instead the country 's electoral system is based on a series of parliamentary acts dating back to the 17th century , when men wore the sort of tights and wigs still donned on ceremonial occasions by some officials of the Houses of Parliament . A general election is when voters nationwide choose lawmakers who will sit in the House of Commons , the lower chamber which initiates and approves legislation . An election must take place , by law , at least every five years . That said , governments can call an election at any time during their term . They could also be forced into an election if they lose a majority of lawmakers in the Commons . Who calls an election ? The starting gun in a general election is traditionally fired when the prime minister travels to Buckingham Palace to ask the queen , the UK 's head of state , to dissolve parliament by royal proclamation . An election must then take place within weeks . Since 1979 , elections have been held between April and June . It is customary for an election to be held on a Thursday . The vote is conducted through the `` first past the post '' system whereby the candidate in each constituency with the most votes wins the seat . If the queen is the head of state , then how come the UK a democracy ? Although the UK is a monarchy , the powers of the reigning head of state have gradually dwindled since King Charles I was beheaded in 1649 following the English Civil War between Royalists and Parliamentarians . Today , the monarch 's role is largely ceremonial and mostly restricted to state functions and appearing on stamps and banknotes . But the monarch still retains formal powers and opens parliament each year . How does parliament function ? The job of parliament is to make laws , scrutinize the actions of the government and debate issues of the day . The Commons -- which has green benches -- currently consists of 650 directly elected lawmakers known as Members of Parliament or MPs who each represent a geographical constituency . The upper house , the Lords -- which has red benches -- is mostly appointed by the government with members serving for life . Its main duty is to vote on legislation passed by the Commons and , when necessary , to send it back for further debate . In certain circumstances the government can force through legislation passed by the Commons but rejected by the Lords . How is an election decided ? Voters do not elect the prime minister , or head of government , directly . Rather , they vote to elect a candidate representing a particular party to serve as their local MP . The leader of the party which wins the most seats is then asked by the Queen to form a government . The leader of the second largest party in the Commons becomes the `` Leader of Her Majesty 's Opposition . '' MPs are elected by a plurality , rather than a majority , of votes . This means that a party can win a majority of seats in the Commons without achieving an overall majority in the overall popular vote . In fact , because there are three main parties in the UK it is extremely rare for a party to win an outright majority . The last prime minister elected by an outright majority was Stanley Baldwin , the Conservative Party leader , in 1931 . In 2005 , Tony Blair 's Labour Party won just 35 percent of the vote but still controlled a 66-seat majority in the House of Commons . So who can I vote for ? For most of the past century , British politics has been dominated by two parties , the left-wing Labour Party and the right-wing Conservative Party . That said , the boundaries between left and right have been blurred as each has strived to occupy the strategically important center ground . A third party , the centrist Liberal Democrats , usually picks up around 20 percent of votes but wins far fewer seats because it is squeezed between the two main parties . Parties from Scotland , Wales and Northern Ireland also send small numbers of lawmakers to Westminster . Full guide to UK political parties What happens if a party does n't win a majority of seats ? If no party wins an overall majority -- a `` hung parliament '' -- - then the leader of the party which takes the most seats will usually be invited by the monarch to form a government . But its powers will be limited because it can not control a majority of votes in the Commons . The party will usually call another election within months in a fresh attempt to secure a majority . What would a hung parliament mean in 2010 ? The last time this happened was in 1974 , when an election in February resulted in a hung parliament . A second election followed in October of the same year . Many opinion pollsters and observers believe that the 2010 general election will result in a hung parliament .
A general election must take place in the UK at least every four years . The power of the monarchy has gradually dwindled since the 17th century . Power in the British parliament rests with the lower house , known as the Commons . The last election where no party had a majority was in 1997 -- analysts predict it will be in 2010 .