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Major isoglosses of German dialects: the Benrath (maken/machen) and Speyer line (Appel/Apfel)

In German linguistics, the Speyer line, Main line (Main river), or Germersheim line is an isogloss separating the dialects to the north, which have a geminated 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. In Germany, the line is best known by its humorous designation Weißwurstäquator.

See: High German consonant shift