Maps of electoral results across the Federal Republic of Germany show remarkable stability over the postwar period. While historical social cleavages are the foundation of western Germany’s stable electoral geography, in the five eastern states crucial events in 1989 and 1990, largely independent of social structural factors, established particular trajectories of party formation that shaped p9arties’ electoral success. Drawing on interviews with political elites and archival documents, I use a critical juncture analysis approach to explain the temporally and spatially contingent nature of party formation processes in eastern Germany after 1989. This analysis shifts debates about party formation from the questions of when and why, to how and where parties form, while also explaining the origin and persistence of territorialized party systems