Speyer line
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 runs from east to west and passes through the town of Speyer.