General elections take place in individual constituencies which each elect one Member of Parliament (MP) to take up their seat in the House of Commons.
The boundaries of these constituencies are redrawn fairly regularly to reflect changes in population and, sometimes, changes in local government boundaries.
This is carried out by an independent Boundary Commission which is required to make the constituencies of roughly the same electorate, except in more remote rural areas as this would make them very large. The Commission looks at social and economic criteria such as transport links and shopping catchment areas.
The changes they recommend can have a dramatic effect on who is likely to win the constituency, if Labour or Conservative voting areas are transferred in or out.
Anyone can object to their proposals and so the parties will do so if they are adversely affected but will do so based on these economic and social criteria.
The Commission considers the objections and makes a final recommendation which is approved by Parliament.
The Commission uses the electorate rather than population to decide the new boundaries and areas where there is under registration will lose MPs.
This particularly affects Labour because more of their MPs are in city areas and areas with a large private rented sector where less people tend to be registered.
The party with the most votes does not necessarily win the most seats.
In 1951 Labour won more votes but the Conservatives won the election because rural seats were smaller and the Conservatives won many of them by small majorities whereas Labour won many larger urban seats with large majorities.
Since the early 1990s there has been a bias towards Labour because inner city seats are a little smaller and turnout is lower in them so that it takes less votes on average to elect a Labour MP.
In 1992 the Conservatives were 7% ahead of Labour which would have given them a large majority in previous elections but their majority was only 21.
Their lead over Labour was also 7% in 2010 but they failed to get a majority at all, though the number of MPs that there are now from other parties also makes achieving a majority more difficult.
The Conservatives included a commitment in the Coalition agreement to reduce the number of constituencies to 600 and make them all almost exactly the same size electorate. This would have corrected the bias in the system but only partly.
When the Conservatives failed to support Liberal Democrat proposals for reform of the House of Lords, the Liberal Democrats failed to let the new constituency system go ahead.
