The map below shows the geographic distribution of German dialects in the first half of the twentieth century.
The Low German (Niederdeutsch) dialect area, which is marked here in blue, extended historically from the eastern Netherlands, across northern Germany into northern Poland and the Russian enclave around the city of Kaliningrad (Königsberg). Much of this territory was part of the kingdom of Prussia (Preußen). Today, there are few German speakers east of the German-Polish border. The major division within Low German is an East-West one. East Low German dialects include Pomeranian (Pommersch), eastern (Ost) varieties of which are widely spoken in Wisconsin. Examples of West Low German dialects include North Low Saxon (Nordniedersächsisch), Westphalian (Westfälisch), and Eastphalian (Ostfälisch).
The thick red linguistic border (isogloss) that divides Low from High German dialects is known as the Benrath Line (named for a small community, now part of Düsseldorf, located near the line). High German dialects, south of the Benrath Line, show at least some effects of what is known as the Second (or High German) Consonant Shift (Zweite Lautverschiebung)
The gray lines running across the map are some of the numerous other isoglosses for sound and word differences that distinguish German dialects from one another.
The large High German area is divided into Central (Mittel) and Upper (Ober) German dialects. The east-west isogloss dividing Central and Upper German is known as the Speyer (or Germersheim) Line. Much of the southeastern Upper German area includes Bavarian-Austrian dialects (Bairisch-Österreichisch). In the southwest, the major dialect area is Alemannic (Alemannisch), which includes Swiss German dialects, Swabian (Schwäbisch), and Alsatian (Elsässisch). The German dialects spoken in far western Austria (Vorarlberg) and the principality of Liechtenstein are also part of the Alemannic group.
Straddling the Upper, Central, and Low German areas are dialects known as Franconian (Fränkisch). Pennsylvania Dutch is a form of Palatine German (Pfälzisch), which is located in the Rhenish-Franconian (Rheinfränkisch) area. The Luxembourgish language is linguistically Moselle Franconian (Moselfränkisch), while modern Dutch is historically descended from Low Franconian (Niederfränkisch).