A hashtag titled #SupremeCourtFacts is trending second on UK Twitter ahead of the appeals, bringing some light relief for those following the hearing.

BBC News Channel

"As in almost everything to do with Brexit, especially the legal questions, we are in unprecedented, uncharted territory," says senior law lecturer Dr Joelle Grogan, from Middlesex University.

Meanwhile, asked what he would do, Robert Craig, of the University of Bristol Law School, says: "If I was one of the 11, I'd be starting with the blistering judgement of the English court which was very senior judges.

"But the Scottish judges were also senior, so it's very balanced."

The organisers of the March for Change protest outside the Supreme Court have tweeted about why the rally is taking place:

BBC Home affairs correspondent Dominic Casciani has shared the timetable of this week's hearings on Twitter:, external

Dominic Casciani
Home Affairs Correspondent

While we’re in the realms of speculation – which is the only place we will be until there is a judgement – let’s have a think about the possible outcomes.

Scenario one: The prime minister wins his appeal against the Edinburgh judgement – and sees off the appeal from London.

That would mean the court has ruled that judges don’t have a power to act as referee in a political-constitutional dispute between the PM and Parliament because, quite simply, there’s no law setting out what a court is supposed to do to resolve the mess.

Scenario two: PM loses Edinburgh appeal and Gina Miller wins hers from London.

That would be devastating for the PM.

Scenario three: PM loses his appeal from Edinburgh – but court upholds his victory in London.

That is theoretically possible as Scotland and England have differing legal systems. The 11 Supreme Court justices will need to provide a clear explanation as to why both positions are right – and why, most importantly, their final decision is the only one that matters.

Are your children asking about Brexit - or do you need a basic recap on what's been happening? (We don't blame you...)

Here's Newsround's explanation of the latest Brexit news.

Image source, PA Media

BBC News Channel

"You can see the strength of the arguments both ways," says public law professor Alison Young, from the University of Cambridge.

"So in one sense, you can see well when we talk about proroguing Parliament, we're thinking about political issues... so you can understand why you think it's a political issue.

"But when you go away and look at the Scottish decision. They're saying, well yes we can see that normally this is about politics but look about what's going on... if you are going to use this for the wrong purpose to undermine aspects of our constitution, then the court should intervene."

She adds: "It's really, really hard to balance those arguments.

"I think we'll understand more of how the Supreme Court is weighing those up when we see the arguments presented and the questions the justices put to the barristers."

Norman Smith
Assistant political editor

It would be an astonishing rebuff for Boris Johnson if - just as he is trying to talk to EU leaders at the UN summit next week - he hears that he is going to have to backtrack and recall Parliament.

I think that will fuel a sense among EU leaders that maybe the prime minister cannot get any sort of deal through Parliament, given the clear opposition he faces there.

But talking to ministers privately they are very confident that the judges will strike down the Edinburgh court, or in other words they will not order Parliament to be called back.

That is because privately they take the view that it would be a massive step for the Supreme Court to intervene in what ministers think is a political decision.

BBC News Channel

Barrister and legal commentator Jeremy Brier says "justiciable" is the buzzword of the day.

"What it means is, it's that threshold of where the courts intervene," he says.

"Is this a matter for judges? Or are there some things in a purely political domain that are the realms of politics, high policy that the courts simply say it's not for us to get involved in?"

The English High Court ruled that the matter was not justiciable, but the Scottish Court of Session ruled differently - and that's why today's so interesting, he adds.

Dominic Casciani
Home Affairs Correspondent

There are no end of stories about what the prime minister will do if the Supreme Court upholds the decision of the Edinburgh Court of Session that the manner in which he closed down Parliament was unlawful. He will certainly face enormous pressure to resign if he is found to have lied to the Queen.

All Mr Johnson has said about today is that we should wait for the outcome. His legal team’s submissions go further. The papers state that if the justices rule against Mr Johnson, “…the prime minister will take the necessary steps to comply with the terms of any declaration made by the court”.

What that means in practice is speculation - because it depends on what the court explicitly says. The justices could tell the PM to “retake” the decision of how he advised the Queen. How and when MPs would then return to work, in that scenario, would obviously be rather unclear.

Today's proceedings get under way in an hour's time. A live stream of from the court will be available to view on this page.

Image source, PA Media

Image source, Reuters

Clive Coleman
BBC legal correspondent

The 11 justices here have to grapple with two completely conflicting rulings:

• The Scottish ruling which ruled that a) this is a matter that the courts can adjudicate on; and b) the prime minister's advice given to the Queen was unlawful because his improper purpose was to stymie, or frustrate, the workings of Parliament in some of those weeks in the lead-up to the UK leaving the EU.
• On the other hand, from the High Court in London, a completely different judgement that said: no, this is a matter of politics, under our constitution the courts cannot get involved in this, there are no legal standards against which to judge the proroguing of Parliament.

It is only the second time 11 justices have sat, that's how important it is - and by the way that's so no one can say if a different panel had sat there might have been a different result.

The 11 justices have to reconcile that - they have to say one of those rulings is wrong.

It's a two-part process. Firstly they have to decide is this a matter for the courts.

If they decide that it is, then they will give a definitive ruling on whether Boris Johnson's advice was unlawful, whether it was given for an improper purpose. It literally could not be more dramatic.

Clive Coleman
BBC legal correspondent

This is massive. We just have to stand back and remind ourselves what's happening here.

We're kind of through the looking glass as far as constitutional law is concerned.

We have to remember we have a prime minister of the United Kingdom who stands accused of misleading the monarch and undermining Parliament, the sovereign body in our constitution.

And that is extraordinary.

These are the 11 Supreme Court justices who will hear the two appeals from today until Thursday.

This is only the second case at the court to be heard by a panel of this size, the first being Gina Miller's challenge as to whether the prime minister or Parliament should trigger Article 50.

Image source, PA Media

Dominic Casciani
Home Affairs Correspondent

There are two fundamental questions that the Supreme Court justices need to grapple with.

First, do judges have the power to interfere with a prime minister’s recognised legal power to advise the Queen to prorogue, or close down, Parliament. This is the fundamental constitutional question at stake and you will hear a lot in the coming days about what’s called the PM’s “prerogative power” – meaning things he can do without the say-so of MPs.

If the court agrees that it can interfere in how he uses that power, the next question is whether he used the power unlawfully by misleading the Queen about why Parliament had to close for five weeks.

Today Programme
BBC Radio 4

While most of the UK's focus today is on London and the Supreme Court, International Trade Secretary Liz Truss is visiting Australia, New Zealand and Japan for talks on post-Brexit trade arrangements.

Former Australian PM Tony Abbott says the UK already does "something like AUD$28bn a year" worth of trade with Australia without a deal.

Australia does AUD$100bn (£55bn) worth of trade a year with the EU without a deal, he adds.

"Let's not get too panicked about the prospect of a no-deal Brexit," he says.

"Britain has actually been doing more trade with the rest of the world than with the EU in recent years."

BBC Home affairs correspondent Dominic Casciani is at court and will be tweeting updates.

Outside the Supreme Court in Parliament Square, Westminster, protesters hold up placards ahead of today's hearings.

Image source, PA Media

Image source, PA Media

Image source, Getty Images

Businesswoman Gina Miller has arrived at the Supreme Court.

She is appealing a judgement by the High Court in London that the suspension of Parliament was not a matter for the courts.

Image source, PA Media

Image source, Reuters