An octagonal church has an octagonal (eight-sided polygon) architectural plan. The exterior and the interior (the nave) may be shaped as eight-sided polygon with approximately equal sides or only the nave is eight-sided supplemented by choir and porch (or narthex) attached to the octagon. This architectural plan is found in some 70 churches in Norway. Among these in Trondheim is the oldest. This type of church plan spread from the Diocese of Nidaros to other parts of Norway. Virtually all octagonal churches in Norway are constructed as log buildings mostly covered by clapboards. Some of the largest churches in Norway are octagonal and the list includes important cultural heritage monuments such as Trinity Church (Oslo), Sør-Fron Church, and Røros Church.