Apart from its roots in the Roman Catholic Church, the origin of the clerical collar is mired in speculation. From theories that trace it to lay apparel, to those that link it to other ecclesiastical garb, the history of the collar dates as far back as the seventh century.
The clerical collar, otherwise known as the Roman collar, was first used by priests in the Roman Catholic Church. Since the Second Vatican Council, in fact, the clerical collar accompanied by ordinary black clothes, has become part of the Catholic canon as mandatory street wear (anywhere outside the church and rectory.) It has since become the accepted dress for the clergy within many denominations of Christianity.
The clerical collar, once it became an official part of clerical dress, was originally composed of a breast piece and rigid collar. The collar is usually held firm by inserting a thin piece of cardboard or leather, and although it has taken modified forms, the rigid collar is traditionally covered by a piece of cloth, held in place by clips, in order to keep it clean.
Although the clerical collar is still a part of the traditional ecclesiastical garb, the origin of the piece of apparel involves much speculation. While some speculate that it was inspired by 16th century lay apparel, the most reliable theories agree that it is a derivative of other ecclesiastical garb. The two pieces most likely to have evolved into the collar are the amice and the pallium. In both instances, it seems as though the collar, whether derived from the amice or the pallium, came into existence as the early Christian church grew in power and saw the need for greater distinctions of rank.
The amice, in essence, is a neckcloth with a broader section to be worn about the head or back of the clergyman, with longer ends to tie around the body. At some point after the 16th century, the longer ends were dropped as part of the amice, leaving only the wide neckcloth. This neckcloth, comprised now of only a single white collar has become the form which we know today. Evidence of the use of the amice during a formal Mass can be traced to the ninth century.
Another theory, dated even earlier than that of the amice, attributes the collar's emanation from the pallium. The pallium, although it is still worn today by the pope, began as a wider piece of cloth that signified the lamb carried across the shoulders of the Good Shepherd. In the seventh century, some accounts recall the use of a narrower version of the pallium, loosely worn about the neck of an archbishop of the Christian church during a missionary effort, to signify that his authority emanated from the pope.