The Columbia Encyclopedia, Sixth Edition.  2001.

pallium

 vestment proper to the pope, who confers it on archbishops in token of their union with and obedience to him. It is a band of cloth worn around the neck and has a 2-in. (5.1-cm) pendant hanging down in both front and back. There are six black crosses on the pallium. It is woven of wool from two lambs presented to the pope at the Church of St. Agnes on her feast day. Certain liturgical functions, such as ordination, require the use of the pallium, and an archbishop may not perform those until he has received it. The pallium is as old as the 6th cent.