How to use the swingometer
HOW SWING WORKS
Swing is a shorthand way for showing the change in the share of the vote, usually between two parties over two separate elections. The calculation is simple.
Step 1. Add the rise in one party's share in the vote to the fall in the second party's share of the vote.
Step 2. Divide your figure by two. The resulting figure is the swing.
Move the pointer to the right for a swing to the Conservatives, or left for a swing to Labour. A swing into the red of Labour, for example, is a swing for the Conservatives and against Labour and vice-versa.
As the pointer moves, the swing will change, with the seats changing colour to the party gaining them.
The overall election result is shown above the swingometer, along with the predicted number of seats in the House of Commons won by each party.
The names of seats predicted to fall with each swing are displayed on the right. Any key figures, such as party leaders or cabinet ministers, at risk appear with their seat.
Click on the highlighted numbers for more information about past elections with similar swings.
There are three different swingometers to chose from. Either select Labour- Conservative, Labour-Liberal Democrat or a Conservative-Liberal Democrat.
Things to remember
The swingometer is not scientific, it just gives an indication of what may happen on particular swings to one party or another.
Note that for a handful of seats on the wheel, a swing to the Liberal Democrats or Conservatives will mean a third party gains that seat.
The swingometer only shows the results up to a 20% swing, so MPs who won their seats with a high majority will not be shown. Prime Minister Gordon Brown, for example, MP for Kirkcaldy and Cowdenbeath, would need a swing of around 45% to unseat him.
This year, many constituencies will be fought on new boundaries designed to balance the electorate in each seat. In order to make fair comparisons with 2005, the swingometer uses "notional" results, worked out by polling experts, which estimate the votes for each party in each new seat as if it had existed in 2005. It does not take account of by-election results since 2005.
Key swings to look out for
1.6% swing against Labour: This is the uniform national swing needed for Labour to lose their overall majority. This means that we would have a hung parliament.
4.3% swing against Labour: This is the uniform national swing needed for the Conservatives to become the largest party. They would still not have an overall majority.
6.9% swing against Labour: This is the uniform national swing needed for the Conservatives to gain an overall majority and therefore form the next government. It would also be largest swing for Conservatives since World War II.
2.5% swing against Labour: This is the uniform national swing needed for the Liberal Democrats to produce a hung parliament.
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More interactive guides
See how vote share for Labour, Conservatives and Lib Dems translates into seats in parliament with the interactive
Check out opinion polls going back to the Thatcher years with the interactive
The BBC Archive introduces
swingometers from the past
and some of the presenters who made the device their own.