"https://www.bbc.com/news/uk-politics-32810887" Article from BBC news
The UK is due to leave the European Union (EU) at 23:00 GMT on 31 October 2019. For those not following every twist and turn, this guide covers the basics.
Brexit - British exit - refers to the UK leaving the EU.
The EU is an economic and political union involving 28 European countries. It allows free trade and free movement of people to live and work in whichever country they choose.
The UK joined in 1973 (when it was known as the European Economic Community). If the UK leaves as planned on 31 October, it would be the first member state to withdraw from the EU.
A public vote - or referendum - was held on Thursday 23 June 2016, to decide whether the UK should leave or remain.
Leave won by 52% to 48%. The referendum turnout was very high at 72%, with more than 30 million people voting - 17.4 million people voted for Brexit.
Theresa May lost the Tory majority after calling an early election in 2017
Brexit was due to happen on 29 March 2019. That was two years after then Prime Minister Theresa May triggered Article 50 - the formal process to leave - and kicked off negotiations. But the Brexit date has been delayed twice.
The UK and the EU agreed a deal in November 2018 but MPs rejected it three times.
The deal consisted of a binding withdrawal agreement - which set out the terms for the "divorce" process - and a non-binding political declaration on the nature of the future relationship between the UK and EU.
The withdrawal agreement covered a range of things including:
the rights of EU citizens in the UK and British citizens in the EU
how much money the UK was to pay the EU (widely thought to be £39bn)
the backstop for the Irish border
The main sticking point for many Conservative and DUP MPs was the backstop.
Currently, there are no border posts, physical barriers or checks on people or goods crossing the border between Northern Ireland and the Republic of Ireland. The backstop is designed to ensure that continues after the UK leaves the EU.
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It comes into effect only if a comprehensive free trade deal is not quickly agreed between the UK and EU. It would keep the UK effectively inside the EU's customs union but with Northern Ireland also conforming to some rules of the single market.
Critics say a different status for Northern Ireland could threaten the existence of the United Kingdom and fear that the backstop could become permanent. But supporters say it is necessary to maintain peace in Northern Ireland.
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Image caption
Boris Johnson, who replaced Mrs May as prime minister, says there are "abundant" technological solutions to the Irish border question
Prime Minister Boris Johnson wants the EU to remove the backstop from the deal. He wants "alternative arrangements" and technological solutions instead.
But the EU has so far refused to change the backstop.
Mr Johnson has said the UK must leave on 31 October, even if that is without a deal.
That means the UK will leave the customs union and single market overnight.
The customs union ensures that all EU countries charge the same taxes on goods coming in from outside. They do not charge taxes on each other's goods. But members cannot strike their own trade deals.
The single market enables goods, services, people and money to move between all 28 EU member states, as well as Iceland, Norway, Liechtenstein and Switzerland, who are members of the European Economic Area. Countries in the single market apply many common rules and standards.
A UK company can sell its product (goods) in Portugal as easily as it can in Portsmouth, bring back the cash (capital), offer maintenance (services) and dispatch a repair team (people).
If the UK leaves the customs union and single market then the EU will start carrying out checks on British goods. This could lead to delays at ports, such as Dover. Some fear that this could lead to traffic bottlenecks, disrupting supply routes and damaging the economy.
Mr Johnson has tried to calm such fears by announcing an extra £2.1bn of funding to prepare for a possible no-deal outcome on 31 October.
If nothing else happens, the UK will leave with no deal on 31 October 2019. But the prime minister says he still wants to leave with a deal on that date. And lots of MPs say they will try to stop the UK leaving with no deal.
Most MPs are against a no-deal Brexit - with the leaders of Labour, the Lib Dems, the SNP, Plaid Cymru and the Green Party voicing their opposition.
One way to prevent a no-deal outcome would be to try to topple the government with a vote of no confidence and replace it with an alternative one that would seek a delay to Brexit, in order to allow a general election or another referendum to take place.
The other way is passing a law to force the government to ask the EU for a delay.
Neither of these routes is guaranteed to succeed.
Brexit could also be cancelled completely by MPs, although few have suggested that they would support that, without the need for the EU's agreement.
A no-deal Brexit could affect individuals in all sorts of different ways.
If the pound falls sharply in response to no deal and there are significant delays at ports, like Dover, it could affect the price and availability of some foods. There are also concerns over potential shortages of medicines, although the government has said much preparation has been done to avoid this.
Most economists and business groups believe no deal would lead to economic harm.
For example, the Office for Budget Responsibility - which provides independent analysis of the UK's public finances - believes a no-deal Brexit would cause a UK recession.
But many Brexit supporters say it is hard to accurately predict what will happen or believe any economic disruption will be short-term and minor.
Image caption
Almost a third of the UK's food comes from the EU
EU citizens in the UK can apply for settled status, allowing them to remain in the country even if there is a no deal. UK expats in the EU are advised to register as residents of the country in which they live.
UK citizens travelling to the EU will need to ensure passports are valid for at least six months on 31 October and will require an international driving permit if intending to use a car.
European Health Insurance Cards (EHIC) will no longer be valid.
Pet passports will also no longer be valid.
A range of other effects and consequences have been discussed.