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Can the Supreme Court strike down laws?
Only Parliament can pass or cancel laws. The Supreme Court's role is to decide on the correct interpretation of those laws when there is a dispute. If the justices think a law conflicts with human rights safeguards, it can tell Parliament it should reconsider the legislation.
Could the Supreme Court rule against the government on suspending Parliament?
The question for the Supreme Court is twofold.
First, do judges have the power to intervene in how a prime minister prorogues Parliament?
And, if they do, was Edinburgh's court right to conclude Mr Johnson had acted unlawfully in closing it for such a long period?
Eleven of the Supreme Court justices - the largest possible panel - will hear legal arguments from the English and Scottish court cases. The government's lawyers will then respond.
The hearing is scheduled to last until Thursday. The only comparable case of this importance took three days. That was Gina Miller’s challenge as to whether the prime minister or Parliament should trigger Article 50 to start the process for leaving the EU
On a simple level, the justices have to decide whether or not the prime minister acted lawfully. If Mr Johnson loses, the question then would be what kind of action the court orders.

If the judges rule that prorogation was legal, Parliament will remain shut until 14 October - unless the government advises the Queen to recall MPs at an earlier date.
Ms Miller's lawyers are, however, asking the justices for a repeat of the declaration from Edinburgh's Court of Session that the PM's actions were "null and void and of no legal effect".
If her side wins, it's still unclear how and when Parliament would resume - whether MPs could simply return to the Commons, or if they would need to be recalled by The Queen on the advice of Mr Johnson.
Either way, Mr Johnson would face enormous pressure to resign. Dominic Grieve QC, the former attorney general and Conservative MP, has said the PM's position would be "untenable" if it is found that he misled the Queen over the reasons for proroguing Parliament.

Who are the Supreme Court justices?
New justices are appointed by the Queen on the recommendation of a panel of legal experts from each of the UK's nations. Applicants must have been a High Court judge for at least two years or a practising lawyer for 15.

Eleven of the court's justices will hear the case against the government. They are:
  • Lady Hale is the president of the court and, before she became a judge, she was the architect of the UK's internationally recognised laws on the rights of children. As the first female justice, she has been critical of the lack of female representation on the body in the past.
  • Deputy president Lord Reed, a senior Scottish judge, also sometimes sits in the European Court of Human Rights and at Hong Kong's Court of Final Appeal. He previously read the Supreme Court ruling that overturned high employment tribunal fees claiming they prevented access to justice.
  • Lord Kerr was Lord Chief Justice of Northern Ireland before joining the Supreme Court at its 2009 opening. As the only Northern Irish justice, he argued for free NHS abortions to be given to Northern Irish women when they travel to England, but the Supreme Court rejected the case.
  • Lord Lloyd-Jones is the Supreme Court's judge from Wales and a Welsh speaker. He specialises in international law and even advised the courts in the trial of former Chilean dictator Augusto Pinochet. He also chaired the Law Commission - which advises the government on which laws need reforming - up until 2015.
  • Lord Wilson is one of the UK's most experienced judges in family law, having spent 12 years ruling on those cases. In 2014, he made the case for extending same-sex marriage to Northern Ireland, arguing that same-sex marriage strengthened the institution rather than weakened it.
  • Lord Carnwath is an expert in company and commercial law and the former Attorney General, or chief legal adviser, to the Prince of Wales. He has a particular interest in environmental law and has attended UN-backed conferences aimed at strengthening legal protections for the environment.
  • Lord Hodge is the second Scottish judge on the panel of 12. He specialises in commercial law, in particular intellectual property and land valuation.
  • Lady Black is another expert in family law who has taught students at university and, from 2010, also sat as a Court of Appeal judge. She was previously the head of International Family Justice - a body which deals with cross-border family cases.
  • Lady Arden was a Court of Appeal judge for 18 years and another former head of the Law Commission.
  • Lord Kitchin is an expert in the complex branch of commercial cases that deals with intellectual property, such as patents that protect an invention being copied by a rival.
  • Lord Sales is the most recently appointed judge, in January 2019, and his varied career includes sitting in the semi-secret tribunal that adjudicates on complaints against MI5. In 2005, he defended the government's position of not holding an inquiry into the Iraq war and was in the original court which
  • ruled government must ask Parliament before invoking Article 50. This led to the Daily Mail labelling him and the other two judges as "enemies of the people."
The only justice not sitting is Lord Briggs, who comes from a career as a judge and lawyer in commercial cases.