Key questions about general election results, and how the BBC reports them, answered.
How does a party win?
By winning more seats in the House of Commons than all the other parties put together. If a party does that, it has what is known as a majority. There are 650 seats available, which means 326 seats are needed to win an overall majority.
However, an effective majority could be smaller as Sinn Fein, which won seven seats in Northern Ireland in 2017, traditionally refuses to swear allegiance to the Queen and, as a consequence, is not entitled to vote.
So in 2017, 650 minus Sinn Fein's seven would be 643 voting MPs, and 322 would have been enough to command a majority. But obviously that figure changes depending on how many non-voting MPs there are.
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.
- What is a majority?
- What is a hung parliament?
- Have constituencies changed since the last general election in 2017?
- What happens in the Speaker's seat?
- What is meant when a party wins, holds or gains a seat?
- What about by-elections?
- What about where MPs have changed parties?
- What is an exit poll and seat forecast?
- What is the cartogram telling me?
- Why don't vote share figures add up?
- How does the BBC calculate turnout?
- Which parties appear in the summary results graphic at the top of the BBC News website?
- Which parties are listed in the overall results table?
- Why does the postcode search not give me the result that I expect?
- What do all the party codes mean?
Follow election night on the BBC
- Watch the election night special with Huw Edwards from 21:55 GMTon BBC One, the BBC News Channel, iPlayer
- As polls close at 22:00, the BBC will publish an exit poll across all its platforms, including @bbcbreaking and @bbcpolitics
- The BBC News website and app will bring you live coverage and the latest analysis throughout the night
- We will feature results for every constituency as they come in with a postcode search, map and scoreboards
- Follow @bbcelection for every constituency result
- From 21:45 GMT, Jim Naughtie and Emma Barnett will host live election night coverage on BBC Radio 4, with BBC Radio 5 live joining for a simulcast from midnight
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 the winning party has got and double the result.
For example, if the winning party has 350 seats what is the majority?
350-325=25. 25x2=50. 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.
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, as when David Cameron's Conservatives signed up with Nick Clegg's Liberal Democrats in 2010.
Alternatively, it can aim to reach agreements with smaller parties to support it in Parliament in the event of a confidence motion aimed at bringing down the government.
This can be in the form of a confidence-and-supply agreement - as with Theresa May's Conservatives and the DUP in 2017. They didn't take ministerial positions but agreed to support the party in confidence votes and budgets.
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.
If no party is prepared to go down one of these paths, Parliament will be dissolved again and there will be another election.
Have constituencies changed since the last general election in 2017?
No. They haven't changed since 2010.
There was a proposal to reduce the number of MPs from 650 to 600 under the coalition government, which would have meant changes to some constituency boundaries.
These have not been implemented.
What happens in the Speaker's seat?
The Speaker of the House of Commons is an MP and has to stand for re-election in his or her constituency at every general election.
Traditionally the biggest parties in the House of Commons do not stand against the Speaker, although the Green Party has done so this year and in other recent elections.
The new Speaker, Sir Lindsay Hoyle, is standing for election in Chorley, Lancashire.
The Speaker is a neutral figure in Parliament, so Sir Lindsay is no longer a member of Labour 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 - Sir Lindsay's seat is regarded as being a Labour constituency, as he won it for the party in 2017 before being elected Speaker.
If Sir Lindsay wins again, the result in 2019 will be described as "Speaker win" - and his seat will be added to the Labour total and not included in any change calculations.
This means the 326 majority target still works, as there is traditionally one deputy Speaker from the same side of the House as the Speaker, and two deputies from the opposing bench.
The votes for Sir Lindsay will be added to our "Others" tally, however, not included for Labour.
The BBC follows the same principle at every general election.
What is meant when a party wins, holds or gains a seat?
What matters most is how many "seats" - constituencies - each party wins, and for things to change in Parliament, 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 mainly fall into two categories: hold or gain.
- Hold: If a party wins a seat that it won in 2017, this is described as a "hold"
- Gain: If a party wins a seat that it did not win in 2017, this is called a "gain". We don't talk about losses in headline results
- Win: If the constituency was under different control when the election was called compared with how it was after the 2017 election we call it a "win". This could be either because the MP elected in 2017 had defected to a different party, or lost a by-election. We do this just so it's less confusing on an individual constituency level, but how it counts on the overall scoreboard is always in comparison with the 2017 result. More on that below
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 five by-elections since 2017 but only one - in Brecon and Radnorshire - resulted in a change of party control.
When the BBC reports general election results, all of these interim by-elections are ignored, to allow for straightforward comparison with 2017's seats.
The BBC uses slightly different terminology to describe results where seats have changed hands in by-elections.
For example, in Brecon and Radnorshire, Liberal Democrat Jane Dodds won from Conservative Chris Davies.
If the Conservative candidate regains the seat on 12 December, the result will be described as a "Conservative win". Likewise if Ms Dodds wins again that will go down as a "Liberal Democrat win".
It would not appear as a net gain in the Conservatives' overall UK seat tally if they win, but if the Lib Dems win that will give them a +1 in the gains total, and the Conservatives -1.
However, if an entirely different party wins a by-election seat, that result will be described as a "gain" from whichever party won the seat in 2017.
It's a similar situation where an MP has changed party but is standing in the same seat as before. More about that below.
The BBC adopts this policy in recognition of the very particular circumstances which often shape the outcome of by-elections.
Comparing seat changes from 2017 is a fairer way of representing how the political expression of voters has altered from general election to general election.
What about where MPs have changed parties?
Despite it only having been two-and-a-half years since the last election, more MPs have changed parties than in any Parliamentary term since 1886 when the Liberals split over the issue of Home Rule in Ireland.
Several Labour and Conservative members unhappy with the Brexit process left to form the Independent Group last February, since then they have been called Change UK and now the Independent Group for Change. They have three candidates standing in this campaign.
Others have joined the Lib Dems or become Independents, and there has also been a considerable number of MPs suspended or expelled from their parties due to internal disciplinary procedures.
For the purposes of calculating change in seats, we will compare these areas with how they were immediately after the 2017 election.
Let's use the case of Chuka Umunna as an example. He won Streatham in London for Labour in 2017, but has been a Liberal Democrat MP since June.
For this election, he's running in the Cities of London and Westminster seat won by the Conservative Mark Field in 2017.
If Mr Umunna wins there this time around, that will count as a Liberal Democrat gain and a Conservative loss, even though he would have been a sitting MP before and after the election.
If the new Labour candidate for Streatham wins, that will be a Labour hold rather than Lib Dem loss.
If the defected MP were to run in the same constituency they won in 2017, that would be treated similarly to the way we deal with by-elections, explained above.
We would describe it as a "win", rather than a hold or gain, for whoever gets the most votes, but for the overall change tallies for the parties we would compare it against the result in the constituency in 2017.
What is an exit poll and seat forecast?
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 company Ipsos Mori for the BBC, ITV News and Sky News. The poll is carried out in Great Britain only, so there are no seat forecasts for the Northern Irish parties.
The results of the exit poll will allow BBC analysts to forecast approximately how many seats each party has won.
What is the cartogram telling me?
Constituencies in the UK are supposed to have broadly similar numbers of people living in them. There are exceptions for island constituencies, like the Isle of Wight (the biggest seat by population) and Na h-Eileanan an Iar (Western Isles) in Scotland, (the smallest).
But in general there are about 70,000 eligible voters in constituencies in England, Scotland and Northern Ireland, and an average of 56,000 in Wales.
Each seat has equal value in the House of Commons, but the way you view them on a map can be quite skewed - rural areas where populations are more spread out take up more space than inner city ones, for example.
The cartogram is a way of viewing each seat as equal size, while attempting to show roughly where in the UK they are.
There are some, mostly around London - whose high population density distorts the map most significantly - that are in different places to where you'd expect, no longer neighbouring the seats they lie next to geographically.
All the seats are correctly within the right nation and English region, but they are occasionally arranged strangely within that.
Why don't vote share changes add up?
On national and constituency scoreboards you may find that if you add all the vote share gains and losses together they don't equal zero, and it's not clear where extra votes have come from.
That's because we only include vote share changes for parties or independents that are running in this election.
So in a constituency where a certain party is not standing this year but they stood in 2017, like UKIP and the Lib Dems in Beaconsfield, the 11% they won between them in 2017 won't be accounted for in the change figures.
The figures for the parties that are running this year will all be correct individually though.
If a party or independent is running this year but didn't run in 2017, their vote share change figure will be the same as their vote share because it is in effect an increase from zero.
The seats change totals will always sum to zero at a national or regional level, even where there are Independents who won seats in 2017 that aren't standing this time, like Lady Sylvia Hermon in North Down.
How does the BBC calculate turnout?
On election night, the BBC calculates percentage turnout by dividing the number of valid votes cast for all candidates by the number of people eligible to vote (known as the electorate).
Spoilt or rejected ballots are not included.
The UK and nations turnout figures are updated through the night, based on electorates divided by valid votes of all constituencies declared so far.
Which parties appear in the summary results graphic at the top of the BBC News website?
When polls close at 22:00 GMT, 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, with the number of votes received as a tie-breaker if they're level on seats.
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".
When we get closer to the end of the night, the predictions will be switched off and we'll just be left with a bar chart summing up the results.
Full UK results are always available via the link to the results homepage, which contains party listings and breakdown of seats, votes and vote share by party.
There are also separate results pages for each of the four nations in the UK, and one for every constituency.
We also publish results for Wales and the 40 Welsh constituencies in Welsh.
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:
- Standing in one-sixth of seats in any UK nation
- Fielding more than five candidates either across the UK or in a single nation
- Achieved greater than 1% of the vote at UK or nation level at the last general election
- Had a sitting MP in the last Parliament
All parties which do not meet these criteria are amalgamated into a group called Others, including the 224 independents running this year.
Each constituency page will always name every party and independent standing in that constituency.
The BBC only recognises parties which are registered as political parties by the Electoral Commission.
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 12 December.
What do all the party codes mean?
Where possible we try to use full party names, but in some places we have to shorten them so they fit better. Some are easier to guess than others, here are what they all mean (ordered by the parties standing the most candidates this time around):
CON - Conservatives
LAB - Labour
LD - Liberal Democrats
GRN - Green Party
BRX - The Brexit Party
IND - Independents (any individual candidate not affiliated to a party registered with the Electoral Commission)
SNP - Scottish National Party
UKIP - UKIP
PC - Plaid Cymru
CPA - Christian Peoples Alliance
YRKS - Yorkshire Party
MRLP - Monster Raving Loony Party
SDP - Social Democratic Party
LIB - Liberal Party
APNI - Alliance Party
DUP - Democratic Unionist Party
UUP - Ulster Unionist Party
SDLP - Social Democratic & Labour Party
SF - Sinn Fein
AONT - Aontú
ANWP - Animal Welfare Party
LBT - Libertarian
ADV - Advance Together
ED - English Democrats
WRP - Workers Revolutionary Party
REN - Renew
AGS - Alliance for Green Socialism
GWLD - Gwlad Gwlad
JACP - Justice & Anti-Corruption Party [The]
SEQ - Socialist Equality
TIG - Independent Group for Change
WEP - Women's Equality Party
YPP - Young People's Party
CHP - Christian Party
CLGB - Communist League
CMU - Communities United
NE - North East Party
PBP - People Before Profit
POS - Proud of Oldham & Saddleworth
PP - The Peace Party
SFP - Scottish Family Party
SPGB - Socialist Party of Great Britain
VPP - Veterans and People's Party
YESH - Yeshua
SPK - Speaker
ASH - Ashfield Independents
BNP - British National Party
BPI - Burnley & Padiham Independent Party
BSJP - Birkenhead Social Justice Party
CF - CumbriaFirst
CMEP - Church of the Militant Elvis Party
CMUK - Citizens Movement Party UK
CP - The Common People
CVP - The Cynon Valley Party
HWDI - Heavy Woollen District Independents
INET - Independent Network
LNIN - Lincolnshire Independents
LUTN - Best for Luton
MK - Mebyon Kernow
MTHR - Motherworld Party
PATR - Patria
PFP - Psychedelic Future Party
RBD - Rebooting Democracy
SHRP - Shropshire Party
SLP - Socialist Labour Party
SNAV - Space Navies Party
TCRP - The Constitution and Reform Party
TLW - Touch Love Worldwide
UGP - The Universal Good Party
WY - Wycombe Independents