'X' is an ancient common-law writ, issued by a court or judge directing one who holds another in custody to produce the person before the court for some specified purpose. Although there have been and are many varieties of the writ, the most important is that used to correct violations of personal liberty by directing judicial inquiry into the legality of a detention. Before the Magna Carta (1215), a variety of writs performed some of the functions of this writ and during the middle ages, the writ was employed to bring cases from inferior tribunals into the king’s courts. Identify 'X'.