Abstract The contrast between Middle High German (MHG) [s] and [] was consistently neutralized to the latter sound after [r] in many modern German dialects, e.g., MHG kirse > New High German Kirsche ‘cherry’. It will be argued that this sound change was a dissimilation of the distinctive feature [high] and that this dissimilation was triggered by an independently motivated OCP constraint banning adjacent consonants with the same value of [high]. Alternative analyses in which the shift from [rs] to [r] is analyzed as a dissimilation of some other feature or as the assimilation of some property will be refuted. The present study also addressed the actuation problem: Why did [rs] shift to [r] in this particular language at this particular time? It will be argued that the structural questions that arise in explaining the [rs] > [r] shift (e.g., Why did [s] shift after [r] but not after other sounds?) as well as specific questions pertaining to the actuation problem derive straightforward answers by considering the phonological system of Middle High German. In particular, one needs to consider the features of Middle High German that were distinctive and which of those distinctive features were active phonologically.