Someone presented the (difficult) challenge of talking about the history of the Roman collar worn by diocesan priests, well here goes:
The Roman collar was unknown as an article of ecclesiastical attire, in its present form, prior to the sixteenth century. And even at that time, it was irregular for religious orders to adopt the practice. It is an embroidered imitation of the turndown shirt-collar of ordinary dress.
In fact, what we now think of as the Roman collar isn’t Roman at all! The singular linen ring is only half of the package, since its original design included two cloth lappets which hung over the shirtfront. By the 18th century, we can even see secular “clerics” wearing the collar (its a clerical collar after all).
But I like what Pope Celestine once said regarding the matter: We [the bishops and clergy] should be distinguished from the common people by our learning, not by our clothes; by our conduct, not by our dress; by cleanness of mind, not by the care we spend upon our person.”
Sources: Ecclesiastical Vestments: Their Development and History, R.A.S. Macalister, M.A., 1896. ; A History of the Mass and its Ceremonies in the Eastern and Western Church, Rev. John O’Brien, A.M., 1886.
A diocesan priest wears the “Roman” collar with a rabat.
A rare instance of a religious in a clerical collar.
St. Marcellin Chapagnat shows us a variant of the old collar with lappets:
An example of an 18th century clerical collar (linen lappets):
The Redemptorists wear a cassock with a modified form of collar: