Johnson and the government argued that the prorogation was a routine oe that ordinarily follows the selection of a new prime minster.
But critics argued that the prorogation was intended to avoid parliamentary scrutiny of the government's brexit plans and to make it harder for parliament to block a no-deal brexit
Supreme court ruled it was unlawful
Arguably introduced/semi-codified a new key constitutional principle as part of parliamentary sovereignty; the accountabilty of the exectuvie of parliament.