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.