This paper examines a set of problems concerning word stress in the substratal Merged Hebrew component in Yiddish. When compared with their historical cognates in Classical Hebrew, the Yiddish words show a stress pattern which appears to conform to the Germanic trochee. The change has frequently been seen as occurring within the history of Yiddish. The present paper demonstrates, however, that (for the relevant Hebrew-origin items) the change from a Hebrew iamb to a trochee necessarily occurred in a period after spoken Hebrew times and before the birth of Yiddish – thus, within one or more intervening Jewish vernaculars. This is demonstrated by consideration of pre-Ashkenazic Hebrew foot structure in light of two historically distinct processes of syncope.