A subpoena ad testificandum is a court summons to appear and give oral testimony for use at a hearing or trial. The use of a writ for purposes of compelling testimony originated in the ecclesiastical courts of Church during the High Middle Ages, especially in England. The use of the subpoena writ was gradually adopted over time by civil and criminal courts in England and the European continent. History The subpoena developed as a creative writ, the "writ subpoena", from the Court of Chancery. Writs of many kinds formed the essential parts of litigation. The primary function of a writ in the 13th and 14th centuries was to convey the king's commands to his officers and servants. It was irrelevant what the nature of those commands might be. The Register of Writs shows a large variety of writs to be administrative in nature, as opposed to judicial. These former writs acquired the name prerogative writs in the 17th and 18th centuries. Prerogative writs that have survived into modern law are the writ of mandamus and writ of certiorari. The medieval writ of prohibition played an important part in the conflict between the church and state in England. The writ was also used in the courts of admiralty and local courts. It has survived in relative obscurity in United States law. The writ subpoena began to be attached to a wide variety of writs in the 14th century. These were an invention of the Court of Equity, which were a part of Chancery. Thus, "subpoena" was a product of the ecclesiastical courts in England. The commonest writ from this era was the Praecipe quod reddat ("You are commanded to return [some misappropriated good or land]"). To these were often added the phrase sub poena ("under penalty"). The development of the writ subpoena is closely associated with the invention of due process, which slowly replaced trial by ordeal. The institution of the jury trial necessitated the hearing of evidence. This, in turn led to the need for a reliable method of compelling witnesses to appear and give testimony. The writ subpoena became the standard method of compelling witnesses. Following the Fourth Lateran Council held in 1215 (overseen by Pope Innocent III who was at the zenith of Papal power), and based on a Latin interpretation of natural moral law, all forms of trial by ordeal or trial by battle were outlawed in