How BBC election results will be calculated and reported in 2017
What elections are taking place in 2017
Elections will be held in 34 councils in England, all 32 councils in Scotland and all 22 councils in Wales on 4 May 2017.
In addition, six areas in England are voting for newly-created "combined local authority mayors".
These mayors will mostly be responsible for economic development in their regions, but some will have powers over transport and housing.
Doncaster and North Tyneside are also electing local authority mayors, who are elected leaders of their respective councils.
Who am I voting for and what do they do?
In local elections, councillors are elected to run local authorities.
A county council (sometimes called a non-metropolitan county) is responsible for running a county, such as Kent or Lancashire.
They look after the largest and most expensive county services including schools, social services, libraries, main roads, public transport, fire and police services, waste disposal and strategic planning.
Some counties are sub-divided into district councils, which look after more local issues and services such as planning and building control, smaller roads, council housing, environmental health, markets, refuse collection, recycling, cemeteries, parks and tourism.
In some places, the two types of council are merged into one to form a unitary authority, which is responsible for all of the services listed above combined.
In 2017, nearly every election in England is for a county council, while all councils in Scotland and Wales are unitary authorities.
What about mayoral elections?
Sometimes called "metro-mayors", six new mayors are being elected to represent several regions in England.
The mayors will serve and represent a number of local authorities in each region.
Their exact powers vary according on the different agreements reached between local authorities and the government, but will mainly cover economic strategy, transport and planning.
Local authority mayors are also being elected in Doncaster and North Tyneside. These mayors act as directly-elected executive leaders of their respective local authorities.
Mayors are elected using the Supplementary Vote system. Voters choose their first and second preferences for mayor.
If a candidate wins 50% of first choices they are elected.
If no candidate wins 50%, all but the top two candidates are eliminated and second preferences from the eliminated candidates are added to their totals.
Whoever has the most votes combined is declared the winner.
Why are only some councils in England up for election?
There are different types of council across the UK and they all hold elections at different intervals.
Not every council has an election every year.
When are results expected?
Some English and Welsh councils will begin counting ballots as soon as polls close at 2200 on 4 May.
Others will start counting on Friday morning with results continuing all day Friday.
Scotland's 32 councils will start their counts on Friday morning, and with first results expected from midday.
Mayoral results will be declared on Friday.
How is council control calculated?
If a party has a majority on any particular council, it is deemed to be in control of that council.
On other councils no group has a majority and these are described as "No Overall Control".
Council control prior to the election has long been defined by the BBC, the Press Association (PA) and others as which party, if any, has a majority on the eve of the poll.
So if a council was won by the Conservatives in 2013, but then through defections and by-election losses became No Overall Control, in 2017 we would describe it as a Conservative gain should the party regain its majority.
How is seat change calculated?
Seat change is based on how many seats each party won at the previous comparable election.
For nearly everywhere in England, the previous election for these seats was in 2013* and for nearly everywhere in Wales in 2012**.
Scotland's last council elections also took place in 2012.
In some councils, boundary changes take place where councils are re-organised and the number of seats on the council changes.
In cases like this, the BBC uses "notional results" to project what the previous result would have been if the new boundaries had been in place at the last election.
The total number of seats per party will be slightly different between the seats-at-dissolution and those won in 2013 and 2012.
* - Doncaster's previous comparable election was in 2015
** - Anglesey's election was delayed by a year until 2013.
What do 'NOC' and 'VAC' and other abbreviations mean?
Party and other abbreviations used in the election are listed below.
ED: English Democrats
LD: Liberal Democrat
LLAIS: Llais Gwynedd
GRN: Green Party
ICHC: Independent Community & Health Concern and Residents
MK: Mebyon Kernow
NOC: No Overall Control
RA: Residents Association
SNP: Scottish National Party
UKIP: UK Independence Party
What councils are up for election this year?
Isle of Wight
Argyll & Bute
Comhairle nan Eilean Siar
Dumfries & Galloway
Perth & Kinross
Neath Port Talbot
Rhonda Cynon Taf
Vale of Glamorgan
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