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