What is "co-cathedral"

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.