title. In any other jurisdiction canonically equivalent to a diocese but not canonically erected as such (prelature, vicariate, ordinariate, prefecture, apostolic administration), the church that serves this function is correctly called the "principal church" of the respective entity—though some have coopted the term cathedral anyway. The Catholic Church also uses the following terms. A pro-cathedral is a parish or other church used temporarily as a cathedral, usually while the cathedral of a diocese is under construction, renovation, or repair. This designation applies only as long as the temporary use continues. A co-cathedral is a second cathedral in a diocese that has two sees. This situation can arise in various ways such as a merger of two former dioceses, preparation to split a diocese, or perceived need to perform cathedral functions in a second location due to the expanse of the diocesan territory. A proto-cathedral () is the former cathedral of a transferred see. The cathedral church of a metropolitan bishop is called a metropolitan cathedral. The term cathedral actually carries no implication as to the size or ornateness of the building, although many cathedrals are impressive edifices simply because diocesan celebrations typically require the capacity of one of the larger churches in the diocese. Thus, the term cathedral is often applied colloquially to large and impressive churches that do not function as cathedrals, (e.g. the Arctic Cathedral in Tromsø, Norway and the Sagrada Família, a minor basilica in Barcelona.). History and organization Origins and characteristics of the first cathedrals The history of cathedrals commenced in the year 313, when the emperor Constantine the Great personally adopted Christianity and initiated the Peace of the Church. Indeed, in strict terminology, there could not have been "cathedrals" before that date, as before the 4th century there were no Christian "cathedrae"; bishops were never seated when leading congregational worship, but instead presided standing on a raised platform or pulpitum. In the third century, the phrase "ascending the platform", ad pulpitum venire, becomes the standard term for Christian ordination. During the siege of Dura Europos in 256, a complete Christian house church, or domus ecclesiae was entombed in a defensive bank, surviving when excavated, in places to wall-top height. The Dura church had been converted out of a large urban courtyard house of standard form, in which two rooms had been knocked together to make an assembly hall, capable of holding 60-75 standing; while a