How election results are calculated and reported
Key questions about general election results, and how the BBC reports them, are answered below.
How do you win?
The simple answer is by winning more seats in the House of Commons than all the other parties put together. There are 650 seats available, which means 326 seats are needed to win an overall majority.
However, an effective majority could be smaller as the speaker and his deputies, although MPs, do not usually vote. Also, Sinn Fein, which won five seats in Northern Ireland in 2010, traditionally refuse to swear allegiance to the Queen, and are not entitled to vote as a consequence.
What is a constituency?
Also called a "seat", in a general election, this is where the political battles are fought.
Voters in each constituency choose just one MP. The constituencies are towns or areas all of roughly the same size.
Have constituencies changed since the last general election in 2010?
After the last election, the Conservatives and Liberal Democrats proposed changing some constituency boundaries and reducing the number of MPs. However, these plans were scuppered by backbench Tory MPs in 2012 and this year's election will be fought in the same seats as 2010.
What happens in the Speaker's seat?
The Speaker of the House of Commons is an MP and has to stand for re-election as Speaker in their constituency at every general election.
Traditionally the biggest parties in the House of Commons do not stand against the Speaker however some parties, such as UKIP, do.
The current Speaker, John Bercow, is standing for election in Buckingham.
The Speaker is a neutral figure in Parliament, so Mr Bercow is no longer a member of the Conservative Party as he was before his election to the role (by parliament).
However, for the purposes of calculating the number of seats belonging to each party - and calculating those held, gained or lost by each party - Mr Bercow's seat is regarded as being a Conservative constituency as he won it for the party in 1997, 2001 and 2005 before being elected speaker.
If Mr Bercow wins again, the result in Buckingham will be described as "Speaker hold" - and his seat will be added to the Conservative total.
The BBC followed exactly the same principle in 2005, when previous Speaker Michael Martin's Glasgow Springburn seat was added to the Labour party's tally.
What is meant when a party wins, holds or gains a seat?
What matters most is how many "seats" each party wins, and for things to change political parties need to win seats from each other.
Because winning seats from each other is so important, a special language is used to show this. Seats that are won can mainly fall into two categories: "hold" or "gain".
Hold: If a party wins a seat that it won in 2010, this is described as a "hold".
Gain: If a party wins a seat that it did not win at the last general election, this is called a "gain".
Win: Where there has been a by-election since the last general election and that by-election resulted in a different party gaining the seat compared to the general election result (see below).
Clearly these are really important to the opposition parties.
If they are to form a new government, they need to win seats from the existing government and other parties to make "gains", while they retain or "hold" all the seats they had last time.
What about by-elections?
By-elections are one-off elections in seats where, for example, the sitting MP has stood down or died. There have been 21 by-elections since 2010.
When the BBC reports general election results, all of these interim by-elections are ignored, to allow for straightforward comparison with 2010's seats.
Some of the terminology used to describe results in by-election seats is different.
For example: In 2010 Labour's Marsha Singh held Bradford West, but resigned in 2012. Respect's George Galloway won the seat in the subsequent by-election.
If Labour regains this seat at the general election, the BBC will describe it as a Labour "win", not a "hold" or "gain". It will not appear as a net gain in Labour's overall UK seat tally.
Another example: In Rochester & Strood, the resignation and defection to UKIP of Conservative Mark Reckless triggered a by-election in 2014. If Mr Reckless retains his seat, the result will also be described as a "UKIP win", but it will appear as a net gain in the UKIP seat tally.
The BBC adopts this policy in recognition of the very particular circumstances which often shape the outcome of by-elections.
Comparing seat change from 2010 represents a fairer way of representing how the political expression of voters has altered from general election to general election.
What is a majority?
To win an election, a party must win enough seats in the House of Commons to form a government. To do that simply one party needs to get one more seat than all the others added together.
That is called an overall majority, but in the shorthand language of elections it is just called "a majority".
There are 650 seats in parliament, so to get one more than everyone else put together a party must get 326 or more to get a "majority".
Of course it makes things much easier for a government if they have many more MPs than all the others put together.
That number is called the "size of the majority. So, if one party were to win 326 seats, then all the other parties added together would be 324.
The majority is therefore 326 minus 324: two. So the smallest majority possible is not one seat but two.
Another quick way of working this out is to take away 325 from the number of seats that winning party has got and double the result. For example:
If the winning party has 350 seats what is the majority?
So the majority is 50.
A tip for any office sweepstake on the size of the majority is never bet on an odd number.
What is a hung parliament?
A hung parliament happens when no single party wins a majority over all the others.
A party can stay in power without an absolute majority by trying to forge an alliance with a smaller party to create a coalition government.
Alternatively, they can aim to reach agreements with smaller parties to support them in parliament in the event of a confidence motion aimed at bringing down the government.
Another possibility is for the biggest party to form a minority government with no agreements with other parties and just try to form majorities in favour of each individual bill as it comes up - an arrangement sometimes called "confidence and supply".
If no party is prepared to go down one of these paths then parliament will be dissolved again and there will be another election.
The last general election in 2010 ended in a hung parliament, with the Conservatives the largest party. After negotiation, a coalition government was formed between the Conservatives and the Liberal Democrats.
What is an exit poll and a prediction?
An exit poll is conducted by approaching voters as they leave polling stations and asking them to fill in a mock ballot paper to indicate how they have just voted.
The exit poll is carried out by polling companies NOP/Mori for the BBC/ITV News/Sky News.
The results of the exit poll will allow BBC analysts to forecast approximately how many seats each party has won.
Results on the BBC website
Which parties appear in the summary results graphic at the top of the BBC News website?
When polls close at 2200, the BBC will broadcast the results of the exit poll. The projected seats for the top five parties based on the exit poll will be displayed in the summary graphic as grey bars.
When the first results are declared, around an hour later, parties will begin to be ordered left to right by number of seats won at that time.
As the night progresses, the summary results graphic will always display the top five parties according to seats won. All other parties' seats will be amalgamated into "Others".
Full results are always available via the link to the results homepage, which contains a listings and breakdown of seats, votes and vote share by party.
When all results are known, the summary graphic will conclude by displaying the top 5 parties by seats won in the new parliament.
In the event of a tie in the top five - eg two parties have two seats - the party with the most votes will be named.
There are no seat predictions available for parties in Northern Ireland, as the exit poll is carried out in Great Britain (GB) only.
Which parties are listed in the overall results tables?
To appear as a named party in either the overall UK scoreboard or a nation scoreboard, a party must fulfil one of the following criteria:
- Is standing in one-sixth of seats in any UK nation
- Is fielding more than 10 candidates either across the UK or in a single nation
- Have achieved greater than 1% of the vote at UK or nation level at the last general election
- Have a sitting MP in the last parliament
All parties which do not meet these criteria are amalgamated into a group called Others.
Each constituency page will always name every party standing in that constituency.
Why does the postcode search not give the result I expect?
The postcode search box uses the latest available data supplied by Ordnance Survey.
Discrepancies can occasionally occur when a postcode search returns a different constituency to the one given on polling cards sent to an address at the same postcode.
Normally the constituencies concerned are next to each other, and it appears these discrepancies occur when postcodes are on the border between the two constituencies.
We would advise people affected to follow the information on their polling card in terms of the constituency they are in and the polling place to be used on 7 May.