Verner's law

Definition
Verner’s Law is a voicing process which affected the Proto-Germanic voiceless fricatives in the intervocal position, so that ɸ, θ, x, xʷ, s became β, ð, ɣ, ɣʷ, z respectively unless there was a stress in Proto-Indo-European on the immediately preceding syllable (in the majority of sources notated as f, þ, χ, χʷ, s becoming ƀ, đ, ǥ, ǥʷ, z). After the application of Grimm's law, Proto-Indo-European voiceless stops should have become voiceless fricatives in Proto-Germanic. However, often they were also voiced. These irregularities to Grimm's law were noticed before Verner: the voicing alternation in certain verb paradigms had been already described by Braune (1874) and was referred to as grammatischer Wechsel after Holtzmann (1870), however, Karl Adolf Verner (1846-1896) was the first to find an explanation for these irregularities. According to his biographical notes (Verner 1903), he formulated the rule already in 1875 and published it in 1876 under a misleading title Eine Ausnahme der ersten Lautverschiebung. Verner's rule can be exemplified with the development of the word *ph2tḗr 'father'. Proto-Indo-European *t developed into ð instead of the expected *θ: Proto-Germanic *faðēr instead of expected *faθēr.