A more thorough explanation:

The Court of Great Sessions in Wales was a common-law court established in 1543 in Wales. It had jurisdiction equivalent to that of the English assizes. The court was bound to follow English law, but not necessarily English case precedent. It was also known as the King's Great Sessions in Wales.

For example, the Court of Great Sessions in Wales operated alongside the English courts and had the same jurisdiction in Wales as the King's Bench and Common Pleas had in England. The court sat twice a year in four circuits, each comprising three counties, and to each circuit were appointed justices 'learned in the laws of this realm'.

In 1830, the Great Sessions were abolished, and England and Wales became one unified jurisdiction. Two extra circuits were added to the English assize system.