It was noted long ago that in some words of Germanic languages we find consonants which do not fit into Grimm’s law, as formulated above. In some cases it is voiced stops, rather than voiceless fricatives, that correspond in Germanic to IE voiceless stops. (Khaimovich p.20)
Let us compare the Latin words frater, mater, pater with their Old English equivalents bropor, modor, f der. By Grimm’s law the sound [t] in all the Latin words should have corresponded to the sound [0] (written p) in all the Old English words. As it was, only the word bropor showed the regular consonant shift [t > 0]. In the two other words we find the voiced stop [d].
The explanation given by the Danish linguist Karl Verner is that the sound quality depended upon the position of the accent in the Indo-European word: after an unstressed vowel the voiceless spirants [f, 0, h] (< [p, t, k]) and [s] were voiced and became finally [b, d, g].
In Sanskrit, where the old Indo-European accent was fairly well preserved, the corresponding words are ‘bhratar, ma’tar, pi’tar. The word ‘bhratar shows that the Indo-European accent was on the vowel immediately preceding the sound [t], therefore the latter was not voiced after changing to [0] in the Germanic languages, while in the words corresponding to ma’tar and pi’tar the sound [t] following an unstressed vowel was voiced after changing to [0] and became first [d] and later [d].
