What makes a church a cathedral has nothing to do with its size or splendor and everything to do with a single piece of furniture: the cathedra, the bishop's throne, which marks the building as the seat of a diocese and the bishop's home church. A small, architecturally modest building can be a cathedral if it holds the bishop's throne, while a vast and famous church elsewhere in the same diocese remains, formally, only a parish church or basilica. This administrative function shaped the cathedral's social role well beyond worship: cathedrals historically hosted diocesan synods, cathedral schools that were medieval Europe's primary centers of learning, and legal and ceremonial functions tied to the bishop's civic as well as spiritual authority. Because cathedrals were frequently built to house the relics of a patron saint, they also became major pilgrimage destinations, their income and building campaigns often driven directly by pilgrim traffic.
Eastern Orthodox cathedrals differ structurally from their Western counterparts, typically centered on a domed crossing rather than a long nave, but they carry the identical administrative logic, marking the seat of a bishop rather than any particular scale of building. A cathedral, in short, is a rank within church governance made visible in stone, and the throne, not the tower, is what confers it.