The Supreme Court is to issue its judgement on how a woman should be defined in law next week.
The case is the culmination of a challenge brought against the Scottish government by a women's group.
But the ruling could have a major bearing how sex-based rights are applied through the Equality Act across Scotland, England and Wales.
For Women Scotland say sex-based protections should only apply to people born female, while the Scottish government says they should also include trans people with a gender recognition certificate.
A ruling is due to be handed down at the court in London on 16 April.
It is expected to set out exactly how the law should treat trans people, and could have implications for the running of single-sex spaces and services.
The long-running legal dispute began with a bill passed at Holyrood in 2018 which aimed to ensure gender balance on public sector boards.
For Women Scotland complained that ministers had included trans people as part of the quotas in that law.
After a series of cases in the Scottish courts, the issue has been sent to the Supreme Court in London for a final ruling.
And it is no longer limited to Scottish law – the judges are deciding on the proper interpretation of the 2010 Equality Act, which applies across Great Britain.
The Act provides protection against discrimination on the basis of various characteristics, including "sex" and "gender reassignment".
The question before the court is what that law means by "sex".