a modification of Grimm's Law accommodating some of its exceptions. It states that noninitial voiceless fricatives in Proto-Germanic occurring as a result of Grimm's law became voiced fricatives if the previous syllable had been unstressed in Proto-Indo-European

an explanation for a series of apparent exceptions to Grimm's law, stating that the Proto-Germanic word-medial voiceless spirants (f, (θ) ; t h) , h, s), derived from the Proto-Indo-European voiceless stops (p, t, k) and voiceless spirant (s), regularly became voiced (v, ; ð) ; th) , g, z), respectively, and final (s) became (z), when the vowel immediately preceding these did not in Proto-Indo-European bear the principal accent of the word