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 the High Middle Ages, especially in England. The use of the subpoena writ was gradually adopted over time by other courts in England and the European Continent.
Read more about Subpoena Ad Testificandum: History, Subpoena As Generally Defined in The United States, Requisites of Form in The United States, Appearance of Writ; Prisoners and Other Detainees; Uniform Rendition of Prisoners As Witnesses, Immunity From Subpoena Service in Civil Cases, Subpoena Power Defined in The Federal Administrative Procedure Act, Related Links