AbstractThe accepted history of Yiddish, based on historical and literary records, places its origin in the Rhineland, in the far west of the German speaking area. The Germanic parts of Yiddish, however, point to dialects much further east. This has been explained through a ‘Principle of Exchangeability’, whereby ongoing contact with other German dialects on the long trek eastward obliterated earlier forms. Unambiguous evidence may be gleaned from the Semitic elements in Yiddish, which have no European cognates. Modern Yiddish dialects show no trace of the old Rhineland merger of Semitic/h/with/h/. The standard model explains this by reversal of merger, and invokes additional reversals to explain the modern Yiddish oppositions/s/vs./Š/, and five long vs. five short vowels in Semitic‐origin items. Reversal of unconditioned merger is untenable. Where seven such reversals are needed to sustain a historical model, it should be abandoned. The comparative method, which takes empirically confirmed modern data as its point of departure, is superior to the writing of language history as a corollary of external or literary h