It is unknown exactly when the pallium was first introduced. Although Tertullian wrote an essay no later than 220 AD titled "De Pallio" ("On the Pallium"), according to the "Liber Pontificalis", it was first used when Pope Marcus (died 336) conferred the right to wear the pallium on the Bishop of Ostia, because the consecration of the pope appertained to him; Pope Symmachus did the same for St. Cæsarius of Arles in 513, and in numerous other references of the sixth century, the pallium is mentioned as a long-customary vestment. It seems that earlier, the pope alone had the absolute right of wearing the pallium; its use by others was tolerated only by virtue of the permission of the pope. We hear of the pallium being conferred on others, as a mark of distinction, no earlier than the sixth century. The honour was usually conferred on metropolitans, especially those nominated vicars by the pope, but it was sometimes conferred on simple bishops (e.g., on Syagrius of Autun, Donus of Messina, and John of Syracuse by Pope Gregory I).