Verner's law

an explanation for a series of apparent exceptions to Grimm's law, stating that the Proto-Germanic word-medial voiceless spirants (f, t̸h, h, s), derived from the Proto-Indo-European voiceless stops (p, t, k) and voiceless spirant (s), regularly became voiced (v, t̸h, 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