Letters patent are instruments issued by the monarch to publicly grant some specific rights to some person(s) with the backing of the royal power. It is also where the word patent (in its intellectual property sense) comes from, since the monopoly of invention was only granted by the authority of the monarch via a letter patent.
Another use of the letters patent is to constituting certain offices, particular those exercising the royal power, for example, the Governor General of Canada.
Under the British system, the courts are thought to be the monarch exercising their power over the bench.
Many High Courts of India were constituted by the British monarch during the colonial rul. The British Parliament allows the monarch to issue letters patent for such purposes in the 1861 High Courts Act. Their jurisdictions were specified by letters patent.
One of such jurisdiction is the power for a High Court to hear appeals from the decisions of one of its justices.
For example, in the letters patent establishing the High Court in Calcutta:
- Appeal from the Court a of original jurisdiction to the High Court in its appellate jurisdiction.- And We do further ordain that an appeal shall lie to the said High Court of Judicature at Fort William in Bengal from the judgment, in all cases of original Civil jurisdiction, of one or more Judges of the said High Court, or of any Division Court, pursuant to Section 13 of the said recited Act: Provided always that no such appeal shall lie to the High Court as aforesaid from any such decision made by a majority of the full number of Judges of the said High Court, but that the right of appeal in such case shall be to Us, Our heirs or successors, in Our or their Privy Council, in manner hereinafter provided.
This clause grants appellate jurisdiction to the High Court over the High Court itself when the court makes a decision in its original jurisdiction (unless the decision was already made by the majority of the court's judges).
Of course, India can establish its own courts now and the power comes from the Constitution (in particular arts. 226 and 227) instead of the monarch personally, although some letters patent remain legally in force. The Indian Parliament now regulates the High Courts according to the Constitution.
But the term letters patent appeal remained, because the appellate jurisdiction came from the letters patent.