A co-cathedral is a cathedral church which shares the function of being a bishop's seat, or cathedra, with another cathedral, often in another city (usually a former see and/or the civil capital). Instances of this occurred in England before the Protestant Reformation in the dioceses of 'Bath and Wells,' and of 'Coventry and Lichfield.' These two dioceses were each named for both cities that served as bishop's seats.
In Belgium, the cathedral of the primatial Roman Catholic Archdiocese of Mechelen-Brussels is the metropolitan St. Rumbold's Cathedral in Mechelen, the archiepiscopal seat. Its co-cathedral is the Cathedral of St. Michael and St. Gudula in Brussels, the state capital. A third, larger church in Koekelberg (also in Brussels) has the status of basilica, not cathedral rank, yet it has received papal visits including a papal beatification.
- The double-titled bishopric of Aire and Dax has a Co-cathedral of Notre Dame in Dax and a Cathedral of St John the Baptist in Aire
- The Archdiocese of Chambéry–Saint-Jean-de-Maurienne–Tarentaise has its cathedral in Chambéry and co–cathedrals in Saint-Jean-de-Maurienne and in Tarentaise
- The bishop of Couserans (a see suppressed in the French Revolution) had two co-cathedral churches at Saint-Lizier
- The bishop of Sisteron (a see also suppressed) had a second throne in the church of Forcalquier, which is still called La Con-cathédrale.
The primatial Metropolitan see of the Roman Catholic Archdiocese of Esztergom-Budapest has its primary cathedral in the old archiepiscopal seat, the Cathedral Basilica of Esztergom, and a co-cathedral basilica in the national state capital Budapest.
- The Diocese of Adria-Rovigo has a Concattedrale di S. Stefano Papa e Martire Concattedrale dedicated to Martyr Pope Stephen I, in Rovigo, which never was a diocese, besides Adria's own Cattedrale di SS. Pietro e Paolo dedicated to Saints Peter and Paul
- The Roman Catholic Diocese of Alghero-Bosa (on Sardinia) has a Marian Co-Cathedral Concattedrale di Beata Vergine Immaculata Concattedrale di Beata Vergine Immaculata, in Bosa besides its cathedral episcopal see, the Marian Cattedrale di Beata Maria Vergine Immaculata Concezione, in Alghero
- The Diocese of Diocese of Amalfi–Cava de' Tirreni has a Marian Co-Cathedral of the Visitan of Mary in Cava de’ Tirreni, besides the cathedral of Apstle Andrew in Amalfi.
- The Diocese of Avezzano has its cathedral in that city and a co-cathedral in its former see Pescina
- The bishopric of Lucera and Troia has its cathedral in the first-named, present see; the second's former cathedral serves as co-cathedral
- The Archdiocese of Sant'Angelo dei Lombardi-Conza-Nusco-Bisaccia has its cathedral in Sant'Angelo dei Lombardi and three co-cathedrals in absorbed former bishoprics : Conza di Campania, Nusco and Bisaccia
- The Diocese of Teggiano-Policastro has a Co-cathedral of the Assumption in Policastro Bussentino and a Marian cathedral of S. Maria Maggiore e S. Michele Arcangelo, in Teggiano
- The Diocese of Termoli-Larino has its cathedral in Termoli and a co-cathedral in Larino
The metropolitan cathedral of the Roman Catholic Archdiocese of Malta is St. Paul's Cathedral in the former capital Mdina. Since the 1820s, the former Conventual Church of St. John in Valletta has been known as St. John's Co-Cathedral.
- The Diocese of Calahorra y La Calzada-Logroño has a Co-Cathedral of St Mary in Logrono besides its cathedral see in Calahora
- Co-cathedral of Santa María de la Fuente la Mayor in Guadalajara, Spain.
- The Diocese of Osma-Soria has a Co-Cathedral of St-Peter in Soria besides its cathedral in Osma
- Latin Diocese of Kyiv-Zhytomyr has a Co-cathedral of the Merciful Father, in Zaporizhia, besides its Kiev cathedral see of Holy Wisdom
- Latin diocese of Kharkiv-Zaporizhia has a Co-cathedral of St. Sophia, in Zhytomyr, besides the Cathedral of the Blessed Virgin Mary in Kharkiv
- The bishop of Simla and Chandigarh has a co-Cathedral, Christ the King Cathedral, at Chandigarh, which never was a diocese, besides the cathedral episcopal see of St. Michael and St. Joseph in Shimla, the capital of Himachal Pradesh state.
- The Roman Catholic Archdiocese of Lingayen-Dagupan has a cathedral in Lingayen, Pangasinan and the original cathedral, now co-cathedral, the Epiphany of Our Lord Parish Church in Dagupan.
- The Melkite Greek Catholic Archeparchy of Homs has in Yabrud (one of the two former eparchials which were title united with the diocese) the co-cathedral of Saints Constantine and Helen, besides its Cathedral of Our Lady of Peace in Homs.
- The Apostolic Vicariate of Anatolia has a Co-Cathedral of St. Anthony of Padua, in Mersin, besides the Marian episcopal see Cathedral of the Annunciation, in İskenderun (Alexandria).
The Latin Apostolic Vicariate of Alexandria had a St. Catherine cathedral in Alexandria and two co-cathedrals: the former cathedrals of the merged-in apostolic vicariates of Heliopolis of Egypt (Our Lady, in that Cairo suburb) and of Port Said (Our Lady and St. Michael in that Sinai Canal port).
Catholic Central America & Antilles
- The Metropolitan Archdiocese of Los Altos Quetzaltenango-Totonicapán has a Concatedral San Miguel Arcángel, in Totonicapán, besides the episcopal see Catedral del Espíritu Santo, in Quetzaltenango.
- The diocese of Zacapa (y Santo Cristo de Esquipulas) has catedral San Pedro (dedicated to saint Peter), in Zacapa and a virtual Co-cathedral, Basílica del Cristo Negro de Esquipulas, which is officially still the cathedral of the (absorbed) Territorial Prelature of Santo Cristo de Esquipulas.
- The Metropolitan Archdiocese of Fort-de-France–Saint-Pierre has a co-cathedral Co-cathédrale Notre Dame de l’Assomption, in Saint-Pierre, besides the cathedral Cathédrale Saint-Louis, in Fort-de-France.
Catholic South America
- The Armenian Catholic Apostolic Exarchate of Latin America and Mexico has a Cathedral Catedral Armênia São Gregório Iluminador, in São Paulo, Brazil and a Co-Cathedral Nuestra Señora de Bzommar, in Montevideo, Uruguay
- The bishop of Primavera do Leste–Paranatinga has a Co-Cathedral São Francisco Xavier, in Paranatinga, besides his episcopal see Catedral São Cristóvão, in Primavera do Leste.
Papua New Guinea
- The Archdiocese of Rabaul has besides its episcopal see -a cathedral in Vunapope- St Francis Xavier’s Co-Cathedral, in the national capital Rabaul.
- The Diocese of Samoa–Pago Pago has a Co-Cathedral of St. Joseph the Worker in Fagatogo besides the Cathedral of the Holy Family in Tafuna, on Tutuila Island
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. 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 its 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). Another is 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 five 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 Basilica of Our Lady of Peace and the Co-Cathedral of Saint Theresa of the Child Jesus, 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, New York.
- Holy Family Cathedral and Our Lady of Guadalupe Co-Cathedral in the Archdiocese of Anchorage, Alaska.
Examples in Canada are:
- the Roman Catholic Archdiocese of Halifax-Yarmouth has the absorbed diocese's former see St. Ambrose Co-Cathedral, in Yarmouth, besides the Metropolitan's own cathedral Minor Basilica: St. Mary’s Cathedral Basilica, in Halifax, Nova Scotia
- the Holy Family Cathedral and St. Paul's Cathedral in the Roman Catholic Diocese of Saskatoon in Saskatchewan.
- Co-Cathedral of Saint-Antoine-de-Padoue in Vieux-Longueuil (suburb of Montréal, Quebec).
- Archdiocese of Galveston-Houston: History of the Cathedral & Co-Cathedral
- http://www.gcatholic.org/churches/data/cathUS.htm Cathedrals in the United States