Yiddish, the language of Eastern-European Jews, has so far been mostly described as Germanic within the framework of the traditional, divergence-based Language Tree Model. Meanwhile, advances in contact linguistics allow for a new approach, placing the idiom within the mixed language spectrum, with the Slavic component playing a significant role. So far, the Slavic elements were studied as isolated, adstratal borrowings. This book argues that they represent a coherent system within the grammar. This suggests that the Slavic languages had at least as much of a constitutive role in the inception and development of Yiddish as German and Hebrew. The volume is copiously illustrated with examples from the vernacular language. With a contribution of Anna Pilarski, University of Szczecin.