The Sewel Convention governs how the UK Parliament legislates in areas of devolved power. In the Supreme Court’s words, it “was adopted as a means of establishing cooperative relationships between the UK Parliament and the devolved institutions.” The Scotland Act 2016 recognized the convention.
In January 2017, in the Miller case, the Supreme Court held that it could not enforce the convention. Judges, it said, “are neither the parents nor the guardians of political conventions; they are merely observers.”