:Verner’s law:

Discovered in 1876 by Karl Verner, the law deals with exceptions to Grimm’s law. A number of words which Grimm’s law predicts should have voiceless fricatives in Germanic languages, actually have voiced ones. For examples, the intervocalic fricative in English father. Verner noticed that all the exceptions were intervocalic and that moreover they were preceded by an unstressed vowel in Proto-Indo-European.