Low Prussian dialect in the context of "Benrath line"

Old Prussian is an extinct West Baltic language belonging to the Baltic branch of the Indo-European languages, which was once spoken by the Old Prussians, the Baltic peoples of the Prussian region. The language is called Old Prussian to avoid confusion with the German dialects of Low Prussian and High Prussian and with the adjective Prussian as it relates to the later German state. Old Prussian began to be written down in the Latin alphabet in about the 13th century, and a small amount of literature in the language survives. In modern times, there has been a revival movement of Old Prussian, and there are families which use Old Prussian as their first language.

Plautdietsch (pronounced [ˈplaʊt.ditʃ]) or Mennonite Low German is a Low Prussian dialect of East Low German with Dutch influence that developed in the 16th and 17th centuries in the Vistula delta area of Royal Prussia. The word Plautdietsch translates to "flat (or low) German" (referring to the plains of northern Germany or the simplicity of the language). In other Low German dialects, the word for Low German is usually realised as Plattdütsch/Plattdüütsch [ˈplatdyːtʃ] or Plattdüütsk [ˈplatdyːtsk], – very often also as Plattdeutsch – but the spelling Plautdietsch is used to refer specifically to the Vistula variant of the language.

Plautdietsch was a Low German dialect like others until it was taken by Mennonite settlers to the southwest of the Russian Empire starting in 1789. From there it evolved and subsequent waves of migration brought it to North America, starting in 1873.