The prorogation case is really complicated - who knew?? - so the judgment may be rather twisty-turny. Here is what to look for:
We're expecting Lady Hale, the president of the court, to give a summary - but even that may be complicated...
1) First off, the justices must decide whether the case against the PM's prorogation of Parliament is "justiciable" - which means "can judges theoretically find that he broke some law or other by closing Parliament for five weeks." The Court of Session in Edinburgh said it was.
2) London's High Court, which is lower, said the opposite. The Supreme Court's 11 justices need to resolve that first question. And if they decide the PM's use of his prerogative, or executive, powers is not justiciable, then it's over. Boris Johnson has won.
3) If the justices rule that prorogation is justiciable, then they need to rule whether the PM's actions were indeed unlawful. They could rule that he did not break the law. That would mean the PM has won. But the court may say something important about how PM's use the power.
4) But if the court rules the PM acted unlawfully - that he overreached his powers and deprived Parliament of its sovereignty - then he has lost. And it becomes a question of "how badly".
5) If they accept the Edinburgh court's judgment - that the PM prorogued Parliament for an "improper purpose" that would be devastating. He would face calls to quit.
6) But they could rule that they're not going to consider his motives (lack of evidence etc) and focus on the consequences of his actions. If they rule that the effect of the PM's decision was unlawful, he loses - but it's then less clear what happens next,
7) The court may make an order setting out what the justices think should happen next. This step is as existential as it comes - it may require a lot of thumbing of a 1797 Act of Parliament about how it can be recalled mid-prorogation.
8) So they are the basic options as we understand them.
