This article scrutinizes the diachrony of relativizers and complement clause subordinators in Russian, Polish, and Czech. Historical morphology indicates a development from agreeing relative pronouns via noninflected relativizers to complement clause subordinators. This concurs with recent findings on Germanic (Axel-Tober 2017), but contradicts more traditional proposals that derive subordinators from demonstratives. The respective syntactic reanalyses are demonstrated on diachronic Slavic corpus data. Moreover, a quantitative comparison of sixteenth- to seventeenth-century East Slavic texts with and without West Slavic interference suggests that the use of kotoryj ‘which’ as a relative pronoun spread into Russian as an inner-Slavic contact-induced change.