In German linguistics, the Speyer line, or Main line (Main river) is an isogloss

Isogloss

An isogloss—also called a heterogloss —is the geographical boundary of a certain linguistic feature, such as the pronunciation of a vowel, the meaning of a word, or use of some syntactic feature...

 separating the dialect

Dialect

The term dialect is used in two distinct ways, even by linguists. One usage refers to a variety of a language that is a characteristic of a particular group of the language's speakers. The term is applied most often to regional speech patterns, but a dialect may also be defined by other factors,...

s 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

Speyer

Speyer is a city of Rhineland-Palatinate, Germany with approximately 50,000 inhabitants. Located beside the river Rhine, Speyer is 25 km south of Ludwigshafen and Mannheim. Founded by the Romans, it is one of Germany's oldest cities...

. In Germany, the line is best known by its humorous designation Weißwurstäquator

Weißwurstäquator

The is a humorous term describing the very different culture and dialect of the southern part of Germany, especially Bavaria, and the rest of the country...

, the white sausage equator.