It’s the day of reckoning in Westminster, as MPs prepare to vote on whether to accept or reject Theresa May’s Brexit deal.
In a day likely to be filled with political jargon, positioning and spin, Sky News takes you clearly through how the run-up to the “meaningful vote” will unfold.
First, it’s May’s sit-down with her cabinet from around 9am
Well before MPs arrive in parliament, Mrs May will chair the weekly Tuesday meeting of her cabinet.
Here, all of her most senior ministers will assemble to discuss issues of the day.
Brexit – and particularly planning for a “no-deal” divorce from the EU – has featured heavily in recent weeks.
There is usually very little official readout from the meetings, but some ministers may begin leaking details, and the Downing Street press office may reveal some details at the twice-daily briefing for journalists.
Then, the Commons will sit at 11.30am
Trying to say with certainty when anything is happening in parliament is almost harder than negotiating the backstop.
But the Commons sits at 11.30am, with two orders of business due first – health questions, which should take around an hour, and a 10-minute rule bill motion – which takes 10 minutes.
Tuesday’s Brexit debate, then, should start around 12.45pm.
MPs who have still not had a chance to reveal how they plan to vote and give their reasons why will finally get that chance, with the debate due to wrap up at 7pm.
We will find out which amendments will be voted on between 10.30am and 11.30am
Before voting on the final deal, MPs will vote on a series of amendments to it.
At least 13 of these have been tabled – by MPs ranging from the Labour leader Jeremy Corbyn, Labour Remain and Brexiteer backbenchers, government rebels and loyalists.
There is no limit on how many can be considered, with Speaker John Bercow granted the power to pick which ones MPs should debate.
That will be made public between 10.30am and 11.30am.
The meaningful vote – between 8pm and 9pm.
Exactly when the vote will be depends on how many amendments are voted on: six were expected to be picked for the scrapped vote in December.
Each one will take around 15 minutes, meaning the vote is expected to take place between 8pm and 9pm.
Is there any way the vote could be pulled?
We are closer to the vote than last time, and Mrs May and multiple cabinet ministers have confirmed it will still go ahead.
But, one of the amendments by senior Labour MP Hilary Benn says both the deal and no-deal should be rejected.
If that is passed then it is hard to see the point in MPs voting on the full deal itself.
It could just die a death on the floor of the Commons.
If the vote does go ahead, how will it happen?
MPs will rise from the green benches in the Commons chamber and exit into one of two corridors.
These are the “aye” and “no” lobbies.
MPs voting for the deal will file down the aye lobby, which runs behind the side of the house where members of the government sit.
MPs voting against the deal will head into the no lobby, which runs behind the opposition side.
They will all give their names to a clerk at the end of the corridor, and register their name with two “tellers” – nominated by the two sides to declare the result.
After 15 minutes, the voting lobby doors will be closed.
MPs will re-take their seats in the chamber, as the tellers line up in front of the Speaker to declare the result.
You can tell before they open their mouths which side has won, as the victors will be standing on the right.
Find out how each MP has declared they will vote here.
What happens if May wins?
If the prime minister has managed to convert the more than 100 Conservative MPs who are planning to vote down the deal, it will be written into law.
That means the UK will leave the EU on 29 March with an agreement.
What happens if she loses?
Under an amendment passed last week, if Mrs May loses she has just three sitting days of parliament to return and present her next steps – or plan B.
The government has announced MPs will not be sitting on Friday, so that makes the prime minister’s deadline the following Monday – 21 January.
Everything else is in the hands of the amendments being tabled tonight.
Some plans include a second referendum, re-negotiating a Canada-style deal with the EU, and setting an end date for the backstop.
Mr Corbyn could also use the defeat to strike and launch a vote of “no-confidence” in the government to force a general election.
Mrs May’s confidence and supply partners the DUP have said they will support her in such a vote, which if every Conservative MP does as well should keep her job safe.
:: Follow and watch the Brexit vote live with a special programme on Sky News from 6-10pm tonight