Bartsch's law was a phonetic change affecting the open central vowel [a] in northern Gallo-Romance dialects in the 5th-6th century.
[1] According to one view, diphthongization took place first, and Bartsch's law is seen as a further segmentation of the diphthong [aɛ] caused by the preceding palatal/palatalized consonant, followed by simplification of the resulting triphthong: According to a second view, Bartsch's law affected the simple vowel [a], causing it to change to [e], which then diphthongized to [ie]: Support for the second hypothesis comes the fact that palatal consonants triggered the same change [a] > [e] in unstressed word-initial syllables:[2] Subsequent changes have obscured the effects of Bartsch's law in modern French.
The accent shifted to the second element of the diphthong [ie], and the first element underwent glide formation: The glide [j] was then lost in most words, either absorbed by the preceding palatal consonant, or eliminated by analogical pressure (e.g. in many verbs of the -er conjugation):[3] The glide was only retained if subsequent nasalization took place, as in Modern French chien [ʃjɛ̃] "dog" (not *chen *[ʃɛ̃] or *[ʃɑ̃]).
[4] Compare with the walloon tchén *[tʃɛ̃] Consequently, the vowel "e" in these words, which is due to Bartsch's law, is now indistinguishable from the "e" that resulted from the general diphthongization of [a] (as in the words mer "sea", porter "carry", mentioned above).
The diphthong [ie] is still visible in the spelling of words like chien "dog" (< canem) and moitié "half" (< Proto-Western Romance [mejˈtate] < Latin medietātem).