This article seeks to explain the synchronic variation found in the second person singular inflectional ending (attested both as -s and -st) in the Old High German Evangelienbuch, while at the same time pro-viding a diachronic account of the introduction and extension of the -st ending in German. In order to achieve these goals, in my analysis I rely on the notions of cliticization and formal analogy, arguing that the innovative and original endings correlate with different syntactic environments (V1/V2 versus Vfinal), on the one hand, and different formal shapes (is versus ôs/ês), on the other. After presenting an account of the development of -st in OHG, I draw conclusions regarding the broader question of how clitics become (part of) inflection, a discussion which in turn has implications for the theories scholars use to describe and explain language change, specifically that of grammaticalization.*