isogloss
geographic boundaries between where linguistic features are used
centum-satem isogloss
Indo-European linguistic classification
Von Wartburg Line
European language border
Speyer line
isogloss separating the Central German dialects (Appel "apple") from the Upper German dialects to the south (Apfel "apple")
Fries Border
The isogloss in the Netherlands which lies around the Rijn and estimates which word a Dutchman would use for fries