Contrastive linguistic studies have focused almost exclusively on contrasting the synchronic grammars of modern standard language varieties. There may be some merit in expanding the scope of the enterprise to include contrastive investigations of grammatical systems with respect to how they change over time. Full understanding of some contrasts between grammars requires reference to the diachronic axis. This paper illustrates the point with one particular case in the Germanic languages, involving parallel instances of reanalysis of relative pronouns as relative complementizers in Yiddish, Afrikaans and Gothic. These parallel changes, operating on grammatical systems with slight differences in initial conditions, yield sharply different outcomes.