This study explores the related concepts of parallel independent development and drift, highlighting in particular the challenge of quantifying isolation. It analyzes the precisely synchronized spread of a sound change, the monophthongization of /aɪ/, across Pennsylvania German ‘speech islands’ in the American Midwest. A key finding is that the intensity and duration of interspeaker contact required to catalyze apparent parallel developments may have lower than expected thresholds. The significance of extensive yet low-intensity cross-migration patterns across these communities at particular points in their histories ultimately leads to an exploration of the minimal level of contact required for diffusion of a change and feeds into recent discussion on the social contexts for transmission and diffusion (e.g., Labov 2007).