Final answer:
The principal church of a diocese with the bishop's throne is called a cathedral. These cathedrals have historically been centers of religious and cultural significance in both the Western and Eastern branches of Christianity, with architectural designs that cater to their ceremonial roles and the circulation of the faithful.
Step-by-step explanation:
The principal church of a diocese, containing the bishop's throne, is called a cathedral. The cathedral is the chief church within a diocese, the area under a bishop's jurisdiction. It sometimes features a distinctive architectural element called a bema, which is the area where the clergy celebrate the Eucharist and the ritual happens.
In the history of Christianity, bishops were pivotal figures within the church's structure. They presided over congregations in their respective cities, and in imperial capitals, these bishops could be called archbishops. The principal churches in the great cities of Rome, Alexandria, and Constantinople, for example, were of significant importance. Over time, the Roman archbishop, who became known as the pope, asserted authority over the entire Western church hierarchy, while the Eastern church maintained its independence.
The role of cathedrals extended beyond their religious functions; they also served as cultural centers and were often designed with purposeful architectural features, such as the clerestory in Western basilicas or the centrally-planned churches of the Eastern Empire. Their designs, such as those seen in pilgrimage churches, facilitated the movement of the faithful and showcased the relics within.