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.
The subpoena developed as a creative writ, the "writ subpoena", from the Court of Chancery

Court of Chancery

The Court of Chancery was a court of equity in England and Wales that followed a set of loose rules to avoid the slow pace of change and possible harshness of the common law. The Chancery had jurisdiction over all matters of equity, including trusts, land law, the administration of the estates of...

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