The pallium is an ecclesiastical vestment in the Roman Catholic Church, originally peculiar to the Pope, but for many centuries bestowed by him on metropolitans and primates as a symbol of the jurisdiction delegated to them by the Holy See. In that context it has always remained unambiguously connected to the papacy. Essentially the same garment is worn by all Eastern Orthodox bishops, and is called omophor.
As early as the 6th century the pallium was considered a liturgical vestment to be used only in the church, and indeed only during Mass, unless a special privilege determined otherwise. This is proved conclusively by the correspondence between Pope Gregory I and John of Ravenna concerning the use of the pallium.
The rules regulating the original use of the pallium cannot be determined with certainty, but its use, even before the 6th century, seems to have had a definite liturgical character. From early times more or less extensive restrictions limited the use of the pallium to certain days.
The symbolic character now attached to the pallium dates back to the 9th century, when it was made an obligation for all metropolitans to petition the Holy See for permission to use it. The evolution of this character was complete about the end of the eleventh century; thenceforth the pallium is always designated in the papal bulls as the symbol of plenitudo pontificalis officii.
In the sixth century the pallium was the symbol of the papal office and the papal power, and for this reason Pope Felix transmitted his pallium to his archdeacon, when, contrary to custom, he nominated him his successor. On the other hand, when used by metropolitans, the pallium originally signified simply union with the Apostolic See, and was an ornament symbolizing the virtue and rank of its wearer.
For his formal inauguration Pope Benedict XVI adopted an earlier form of the pallium, from a period when it and the omophor were virtually identical. It is wider than the modern pallium although not as wide as the modern omophor, made of wool with black silk ends, and decorated with five red crosses, three of which are pierced with pins, symbolic of Christ’s five wounds and the three nails. Only the Papal pallium was to take this distinctive form. Beginning with the Solemnity of Saints Peter and Paul Benedict XVI reverted to a form similar to that worn by his recent predecessors, albeit in a larger and longer cut and with red crosses, therefore remaining distinct from pallia worn by metropolitans.
At present only the Pope and metropolitan archbishops wear the pallium. A metropolitan has to receive the pallium before exercising his office in his ecclesiastical province, even if he was previously metropolitan elsewhere. No other bishops, even non-metropolitan archbishops or retired metropolitans, are allowed to wear the pallium unless they have special permission.