Duchy of Franconia
Throughout the Middle Ages, the marked the boundary of the Duchy of Franconia with the Landgraviate of Thuringia. Even today it forms a clear border between the Franconian part of South Thuringia from the otherwise Thuringian–Upper Saxon lands of Thuringia. The as a language border separates the East Franconian dialects (', ' and "Upper Franconian") from the Thuringian dialects ("Central Thuringian", " Thuringian" and "Southeastern Thuringian") spoken in the mountains, in the northern part and east of the Thuringian Forest.