Minority speech varieties often retain insightful archaisms and innovations which are absent from the diachronic descriptive grammars of major national languages. One such innovation is the splitting of the Proto-Romance numeral adjectives ūnu ‘one.masc’ and ūna ‘one.fem’ into phonologically distinct tonic numerals on the one hand and cliticised indefinite articles on the other. Employing recent dialectal data collected at Saint-Pierre-le-Bost (Dubac & Quint 2021) in France’s Creuse department, we employ autosegmental strict CV representations (Scheer 2004) to account for the numeral’s development from Latin into the phonologically differentiated numerals and articles of the Gallo-Romance Croissant. Under stress, the inherent palatal quality of the initial vowel spread into the empty leftward C position; the numerals thereby underwent yod insertion in onset. When cliticised the same numerals resulted in reduced forms suggesting that the lexical split between tonic numerals and cliticised articles began as a stress-dependent phenomenon. This evolution was hitherto left unexplained in descriptions of Croissant varieties. Moreover, the formal analysis of this phenomenon contributes to our understanding of grammaticalisation and dialect formation through parameter setting more broadly in Romance.