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In German dialectology, the Speyer line or Main line (Main river) is an isogloss separating the Central German dialects to the north, which have a stop in words like Appel "apple", from the Upper German dialects to the south, which have an affricate: Apfel. The line begins in Alsace near Strasbourg, and runs north-east to Thüringen, crossing the Rhine at Speyer. After passing close to Erfurt, it turns south-east and continues into the formerly German-speaking parts of Bohemia. It can be observed well by the fact that place-names containing an uncombined /p/ phoneme mostly lie to the north of the line (Paderborn, Potsdam, Wuppertal), while those with an affricate /pf/ (Pfaffenhofen, Pforzheim) mostly lie to the south.
- Rolf Bergmann; Claudine Moulin; Nikolaus Ruge (2019). Alt- und Mittelhochdeutsch: Arbeitsbuch zur Grammatik der älteren deutschen Sprachstufen und zur deutschen Sprachgeschichte (in German) (10th ed.). Göttingen: Vandenhoeck & Ruprecht. p. 112.