Prime Minister Boris Johnson's case that he acted lawfully over the suspension of Parliament for five weeks is under the spotlight at the UK's highest court.
The Supreme Court is hearing two appeals of two earlier rulings in the English and Scottish courts, in which leading judges reached different conclusions.
At the High Court in London, judges rejected a challenge against the prorogation by campaigner and businesswoman Gina Miller, but in Scotland, a cross-party group of MPs, won a ruling that Mr Johnson's suspension of Parliament was unlawful.
Now, a panel of 11 Supreme Court judges must decide whether it's right for the courts to rule on disputes between Parliament and the Government and if so, they will rule whether Mr Johnson's acted lawfully when he suspended Parliament.
Here's everything you need to know about the highest court in the land.
What is the supreme court?
The Supreme Court is the highest court in the land, and the final court of appeal for all United Kingdom civil cases and criminal cases from England, Wales and Northern Ireland.
It was established so there was complete separation between Parliament and the UK's highest judges - know as justices.
The House of Lords had been the final court of appeal until 2009.
The Supreme Court is housed in the former Middlesex Guildhall - opposite the Houses of Parliament.
What cases have been heard in the Supreme Court?
Matters are initially heard in lower courts and then if a party disagrees with the legal reasoning and outcome of the case they can follow a route of appeal that ends at the Supreme Court.
Only cases of the "greatest public and constitutional importance" will be heard in the Supreme Court.
The UK Supreme Court has decided many important cases on a variety of legal issues in the last ten years.
Some examples include the Supreme Court ruling the Government acted unlawfully and unconstitutionally when it introduced employment tribunal fees for those bringing claims.
As a result of the ruling, the Government had to take immediate steps to stop charging the controversial fees of up to £1,200 for taking a case to tribunal.
Gina Miller launched a legal challenge against the Government's decision to trigger Article 50 - starting a two-year countdown to the UK’s departure from the EU - without consulting Parliament.
The High Court ruled the prime minister did not have the power to trigger Article 50 without the authority of Parliament, a ruling ultimately upheld by the Supreme Court in January 2017.
In October 2018, the Supreme Court backed a Northern Ireland bakery that refused to make a gay marriage cake.
The UK's highest court ruled the Belfast bakery, Ashers, was not obliged to make the cake with the message “support gay marriage”.
The five justices said the bakery did not refuse to make the cake because of his sexual orientation and therefore there was no discrimination on those grounds.
What is the structure of the UK courts?
All criminal courts start in the magistrates' court, but more serious matters will be sent to the Crown Court.
Any appeals will then go to the High Court, potentially the Court of Appeal and the final court of appeal - the Supreme Court.
Civil cases will sometimes be dealt with by magistrates, but may well go to a county court.
Again, appeals will go to the High Court and then to the Court of Appeal – although to different divisions of those courts.
Who are the justices in the Supreme Court?
An odd number of justices (judges) always sit in the Supreme Court, in case there is a split decision, ensuring a decision is always made.
There are a total of 12 Supreme Court justices, but 11 are taking part in the three-day hearing on deciding if the prime minister acted lawfully.
The court comprises of the president and deputy president and 10 other judges of the Supreme Court.
Lady Hale, president of the Supreme Court, she has presided over the court for two years.
Lord Reed, deptuy president of the Supreme Court, he was first appointed a justice in 2012 and previosuly had served as a judge for 13 years.
Lord Kerr, was the Lord Chief Justice of Northern Ireland before he joined the Supreme Court when it opened in 2009.
Lord Wilson, is an expert judge in family law, was a judge of the Family Division of the High Court. Until May 2011, he was a judge of the Court of Appeal, before he joined the Supreme Court.
Lord Carnwath, before joining the Supreme Court he was Senior President of Tribunals and served as Attorney General to Prince Charles and is an expert in family law.
Lord Hodge, is one of the two Scottish Justices of the Supreme Court and joined in 2013.
Lady Black, was the second woman to joint the Supreme Court in 2017 and is also an expert in family law.
Lord Lloyd-Jones, is the first justice of the Supreme Court to come from Wales and is a Welsh speaker.
Lady Arden, was the third female justice of the Supreme Court when she joined in 2018 and had been a Court of Appeal judge for 18 years.
Lord Kitchin, joined the Supreme Court in 2018 and has worked in the chancery division.
Lord Sales, is the latest justice to be appointed to the Supreme Court in January 2019, and is the youngest member.
Lord Briggs, use to be a judge and a lawyer in commercial cases, but is the only justice not sitting in the Government hearing.