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, s]]. 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. The Supreme Court of the United Kingdom is the highest court in the land for all criminal and civil cases in England and Wales and Northern Ireland, and for all civil cases in Scots law. The Supreme Court came into being in October 2009, replacing the Appellate Committee of the House of Lords. In England and Wales, the court system is headed by the Senior Courts of England and Wales, consisting of the Court of Appeal, the High Court of Justice (for civil cases) and the Crown Court (for criminal cases). The Courts of Northern Ireland follow the same pattern. In Scotland the chief courts are the Court of Session, for civil cases, and the High Court of Justiciary, for criminal cases. Sheriff courts have no equivalent outside Scotland, as they deal with both criminal and civil caseloads. The Judicial Committee of the Privy Council is the highest court of appeal for several independent Commonwealth countries, the British overseas territories, and the British Crown dependencies. There are also immigration courts with UK-wide jurisdiction — the Asylum and Immigration Tribunal and Special Immigration Appeals Commission. The Employment tribunals and the Employment Appeal Tribunal have jurisdiction throughout Great Britain, but not Northern Ireland.