The United Kingdom has three legal systems.English law, which applies in England and Wales, and Northern Ireland law, which applies in Northern Ireland, are based on common-law principles. Scots law, which applies in Scotland, is a pluralistic system based on civil-law principles, with common law elements dating back to the High Middle Ages. While England and Wales, Northern Ireland, and Scotland diverge in the more detailed rules of common law and equity, and while there are certain fields of legislative competence devolved in Northern Ireland, Scotland, Wales and London, there are substantive fields of law which apply across the United Kingdom.
The United Kingdom does not have a single legal system since it was created by the political union of previously independent countries. Article 19 of the Treaty of Union, put into effect by the Acts of Union in 1707, created the Kingdom of Great Britain but guaranteed the continued existence of Scotland's separate legal system. The Acts of Union of 1800, which combined Great Britain and Ireland into the United Kingdom of Great Britain and Ireland, contained no equivalent provisions but preserved the principle of separate courts to be held in Ireland, of which the part called Northern Ireland remains part of the United Kingdom.