The House of Lords' ability to consider judicial appeals (its "appellate jurisdiction") went back to the 12th century when the Monarch dispensed justice via the "Curia Regis" (an advisory council of landowners and clerics).
Although separate common law courts later developed, the "High Court of Parliament" remained the highest court of royal justice, and when the House of Lords emerged as a separate House in the late 14th century it gradually assumed that role.
The judicial work was carried out by Members of the Lords with legal experience and expertise but also by lay Members of the House (those without judicial experience), and this remained the position for many centuries.
While the judicial work of the Lords declined in the 16th century, the jurisdiction of the House was recognised in both legal treatises and statute, and it also constituted a court in certain trials, including those of Members of the Lords and in impeachment cases.
From the early 17th century the number of petitions seeking justice increased - both as a result of proceedings in a lower court and when a petitioner took his complaint directly to the House - and in 1621 the House appointed a Committee for Petitions to accept or reject petitions.