Westminster is the heart of British government. It's where parliament is based and politicians meet to decide laws and make decisions about how the UK is run.
1. House of Commons
The House of Commons is the most powerful of the two houses of parliament. After the election, it will be made up of 650 MPs, almost all representing political parties and each representing different constituencies. Policies and laws are debated here - although some powers have been devolved to Scotland, Wales and Northern Ireland and some laws are made in Europe.
2. Other parties and independents
Apart from the three main political parties, there are 10 other smaller groupings represented by MPs, including the Scottish National Party and Plaid Cymru, as well as a number of independents. Sinn Fein has several MPs but they do not vote as they haven't taken their seats in the Commons. Independents can vote with whichever party they like.
The opposition is usually the second-biggest party in parliament. Since 1997 this has been the Conservatives. Twenty days in each parliamentary session are set aside for opposition debates - 17 are usually taken by the Tories and three by the Liberal Democrats. The opposition leader challenges the prime minister during prime minister's questions when he can ask six questions.
4. Liberal Democrats
The Liberal Democrats are currently the third biggest political party represented in the Commons. Like the Conservatives, they also have a shadow cabinet made up of MPs representing the main departments. Twenty days are set aside for opposition debates in each parliamentary session - usually 17 are allotted to the Tories and three to the Liberal Democrats.
5. Leader of the Opposition
The person in charge of the largest opposition party becomes the official leader of the opposition. There's also an opposition leader in the Lords. The opposition leader in the Commons picks members of the shadow cabinet. Each member of the shadow cabinet scrutinises a different government minister's department and devises policies in that departmental area.
The party which wins more than half of the seats in an election forms the government which will run the country. The party's leader becomes prime minister and chooses which MPs will take the lead roles in different departments like justice and education. About 100 will become ministers and form the government. If no party wins more than half the seats this is called a
. Parties can then unite to form a coalition government.
7. Prime Minister
The leader of a political party that wins more than half the seats in the Commons wins the right to be prime minister. The prime minister also chooses who he wants in the Cabinet and has meetings with the Queen. The prime minister answers questions every Wednesday in the House of Commons. Questions can come from any MP and on any subject.
The Speaker is the chief officer of the Commons and is elected by MPs. He takes charge of debates in the Commons, choosing which MPs should speak and calling MPs to "order" when debates become unruly. Although he or she is usually a member of one of the main parties, once elected the speaker puts party politics aside and has to act impartially.
The cabinet is the central decision-making group of government made up of about 20 ministers chosen by the prime minister from the Commons and Lords. Cabinet members have a collective responsibility, which means they all have to support the decisions which are made. The shadow cabinet is made up of the opposition's key spokesmen for the main departments.
Backbench MPs don't hold ministerial or shadow cabinet posts and so they aren't as strictly bound by loyalty to the government or front bench and can speak more freely and even vote against the government if they wish. They can sometimes introduce their own ideas for legislation in the form of a private member's bill. Backbenchers are so-called because they sit on the back benches.
11. House of Lords
Members of the House of Lords are not publicly elected. Most are appointed by being created life peers - they make up 587 out of 704 members (excluding some through absence etc). There are 92 hereditary peers, who are entitled to sit in the chamber because they hold an inherited title such as duke, marquess, earl, viscount or baron. The main job of the Lords is to scrutinise proposed new laws to make sure they are fair and workable. Some government bills will be sent to the Lords for their first reading to spread the workload between the two houses.
One of the key roles of parliament is to make and change the laws governing the UK. Both houses of parliament generally have to agree on a new law - after a process which can take months, or even years. The key stages of the process are outlined below.
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Plans of the House of Commons and House of Lords based on information from www.parliament.uk
Take a tour of Westminster with Black Rod
Sir Freddie Viggers - Black Rod - gives the BBC an exclusive look behind the scenes.
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