A co-cathedral is a cathedral church which shares the function of being a bishop's seat, or cathedra, with another cathedral. Instances of this occurred in England before the Protestant Reformation in the dioceses of Bath and Wells, and of Coventry and Lichfield, hence the names of these dioceses. In France the bishop of Couserans (a see suppressed by the French Revolution) had two co-cathedral churches at Saint-Lizier, and the bishop of Sisteron (a see also suppressed) had a second throne in the church of Forcalquier which is still called La Con-cathédrale.
In Belgium, the cathedral of the Roman Catholic Archdiocese of Mechelen-Brussels is the metropolitan St. Rumbold's Cathedral in Mechelen; however, its co-cathedral is the Cathedral of St. Michael and St. Gudula in Brussels.
In the case of York the collegiate churches of Beverley, Ripon and Southwell were almost in the same position, but although the archbishop had a stall in each he had no diocesan cathedra in them, and the chapters were not united with that of the metropolitan church in the direct government of the diocese, or the election of the archbishop, nor had they those other rights which were held to denote the cathedral character of a church.
In the United States, there are several instances in which a Roman Catholic diocese maintains two Episcopal See cities, each with their own cathedral or co-cathedral. Examples include the Cathedral of Saint Paul and the Basilica of Saint Mary in the Archdiocese of Saint Paul and Minneapolis (Minnesota) and St. Mary's Cathedral Basilica and the Co-Cathedral of the Sacred Heart in the Archdiocese of Galveston-Houston (Texas).
Other instances include the Diocese of Altoona-Johnstown (Pennsylvania), the Diocese of Fort Wayne-South Bend (Indiana), the Diocese of Great Falls-Billings (Montana), the Diocese of Houma-Thibodaux (Louisiana), the Diocese of Kansas City-Saint Joseph (Missouri), the Diocese of Pensacola-Tallahassee (Florida), the Diocese of Springfield-Cape Girardeau (Missouri), and the Diocese of Wheeling-Charleston (West Virginia).
There are also four instances in the United States in which a cathedral and its co-cathedral are in the same city. This usually occurs when a historically important cathedral becomes too small to serve a growing population, and a larger co-cathedral is constructed to accommodate larger services. Examples include:
- The Cathedral of Our Lady of Peace and the Co-Cathedral of Saint Theresa of the Child Jesus, both within walking distance of each other in the Diocese of Honolulu (Hawaii).
- The Cathedral of the Immaculate Conception and the Co-Cathedral of Saint Joseph in the Diocese of Burlington (Vermont).
- The Cathedral of Mary Our Queen and the Basilica of the National Shrine of the Assumption of the Blessed Virgin Mary in the Archdiocese of Baltimore (Maryland).
- The Cathedral Basilica of St. James and the Co-Cathedral of St. Joseph in the Diocese of Brooklyn
- http://www.archgh.org/cocathedral/dedication/history-cocathedral.htm Archdiocese of Galveston-Houston: History of the Cathedral & Co-Cathedral
- http://www.gcatholic.org/churches/data/cathUS.htm Cathedrals in the United States