Abstract
Late medieval High German experienced a systematic sound shift, although not consistently reflected in modern standard German, which transformed /rs/ into /rʃ/. It is argued that this sound shift must be analysed phonetically as a perseverative tongue shape and place of articulation assimilation from [rs] to an apical retracted alveolar [rs̠̺], followed by the phonological reanalysis of a perceptually ambiguous [rs̠̺] as /rʃ/. This analysis implies the hypothesis that only alveolar rhotics can trigger s‑retraction in /rs/, not uvular rhotics. Approximatively seven hundred years after the MHG s‑retraction, a similar sound shift occurs in Flemish varieties of standard Dutch. A pilot study with Flemish speakers confirms that the motivation for s‑retraction in /rs/ must be attributed to an alveolar /r/. Further typological inquiries into the historical phoneme inventories of English, Dutch and German varieties, reinforced by the comparison with other (non‑)Germanic languages, also confirm this hypothesis.