Major isoglosses of German dialects: the Benrath (maken/machen) and the Speyer line (Appel/Apfel)
In German linguistics, the Speyer line, or Main line (Main river) is an isogloss separating the dialects to the north, which have a geminated (lengthened) stop in words like Appel "apple", from the 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.