This paper considers the coherence of the German consituent of Yiddish. Before the 16th century, several Jewish authors from western German-speaking regions employed languages quite similar to local Christian dialects. In the16th and 17th centuries two separate western and eastern Jewish idioms existed. The former comprised western Germany and northern Italy and was chiefly grounded on East Franconian and Swabian. The latter distinguishes works written in Bohemia and Poland. Southwestern Yiddish receives much East Franconian traits, whereas Eastern Yiddish is mainly based on Bohemian. Its consonantal system got later adjusted to the Silesian dialect of the German Christian urban citizenry in Polish towns. Thus all modern Yiddish varieties do not result from one single hypothetic Proto-Yiddish language.