What is the X in X-mas?
square cap worn by Catholic clergy, 1590s, from Italian beretta, from Late Latin birrus, birrum "large cloak with hood;" perhaps of Gaulish origin, or from Greek pyrros "flame-colored, yellow."
stiff square hat with three or four rounded ridges, worn by Roman Catholic, some Anglican, and some European Lutheran clergy for both liturgical and nonliturgical functions. A tassel is often attached. The colour designates the wearer's rank: red for cardinals, purple for bishops, and black for priests.