Each diocese has a see city (or cities) that give the diocese its name. Over the course of history, see cities are transferred to other cities, especially as populations shift and demographics in a given diocese change. Some diocesan sees are the victims of historical circumstances such as enemy invasion of Christian lands. Those bishops who were exiled retained the “title” of their old diocese, part of their own title of office.
But, as it goes, dioceses that relocate sees aren’t totally defunct; they technically are “suppressed.” There seems to be an in-built hope that the Christian population of that particular defunct see will return and find its status restored as a see city once again.
In the United States, there are several cities that no longer function as diocesan see cities. These include: Allegheny, Pennsylvania; Alton, Illinois; Bardstown, Kentucky; Concordia, Kansas; Grass Valley, California; Jamestown, North Dakota; Kearney, Nebraska; Lead, South Dakota; Leavenworth, Kansas; Natchez, Mississippi; Natchitoches, Louisiana; Nesqually, Washington; Quincy, Illinois; Sault Ste. Marie, Michigan; Vincennes, Indiana and Walla Walla, Washington.
In the Catholic Church’s understanding of the office of bishop, it is important for a bishop to be attached to the People of God in a given place, even if they don’t live there. So when bishops are ordained for a purpose other than serving as the diocesan bishop, they are given the title of one of these “dead” see cities — also known as titular sees.
Those bishops who receive titular sees in the Church today function in one of the following capacities: cardinals or bishops who are nominated as bishop to serve in the Roman Curia without having been attached to another diocese, nuncios, apostolic delegates and other dignitaries of the Curia (typically so, unless they were already diocesan bishops), coadjutors and auxiliary bishops, and bishops who resign their dioceses (although not always the case).
Michael R. Heinlein is editor of Simply Catholic. Follow him on Twitter @HeinleinMichael.