Discovered by the Dane Karl Verner in 1875 (published in 1877), an exception to the Germanic sound shift (Grimm’s law) that was later designated as a ‘law’ by linguists. Based on comparative studies of Sanskrit and Greek with the Germanic dialects, Verner recognized that the placement of free word stress in Indo-European was the cause for apparent irregularities in the consonantism of etymologically related words which Grimm had dubbed ‘grammatical alternation.’ According to Verner’s observation, the Germanic voiceless fricatives [f, θ, χ, s] resulting from the Germanic sound shift became, in the proto-Germanic period, the corresponding voiced fricatives in medial and final position when in a voiced environment, if the immediately preceding vowel did not carry the main stress; cf. IE Goth. fadar (‘father’) in contrast to OInd. Goth. broþar (‘brother’). In the derivation of father the IE/Grk t developed into a voiced fricative (Goth. ), since the stress lay behind the dental, while in brother the IE/OInd. t, according to the Germanic sound shift, was shifted to a voiceless fricative. Phonetically, this sound change can be plausibly explained by differences in air pressure according to the position of the stress; phonologically it is a matter of phoneme splitting (sound change), that takes place when the free stress in Germanic is fixed on the root syllable since, at that point in time, the original (allophonic) complementary distribution was suspended.
in medial and final position when in a voiced environment, if the immediately preceding vowel did not carry the main stress; cf. IE 
Goth. fadar (‘father’) in contrast to OInd. 
), since the stress lay behind the dental, while in brother the IE/OInd. t, according to the Germanic sound shift, was shifted to a voiceless fricative. Phonetically, this sound change can be plausibly explained by differences in air pressure according to the position of the stress; phonologically it is a matter of phoneme splitting (
