Verner's law

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.

References

• Beekes, Robert S. P. 1995. Comparative Indo-European Linguistics: An Introduction. Amsterdam: John Benjamins.
• Braune, Wilhelm. 1874. Über den grammatischen Wechsel in der deutschen Verbalflexion. Beiträge zur Geschichte der deutschen Sprache und Literatur 1.513-27.
• Collinge, Neville Edgar. 1985. The laws of Indo-European. Amsterdam: John Benjamins.
• Holtzmann, Adolf. 1870. Altdeutsche Grammatik I. Leipzig: Brockhaus.
• Lehmann, Winfred P. 1967. A reader in nineteenth century historical Indo-European linguistics. Bloomington: Indiana Univ. Press.
• Ringe, Donald A. 2006. A History of English: Volume I: From Proto-Indo-European to Proto-Germanic. Oxford: Oxford University Press.
• Rooth, Erik 1974. Das Vernersche Gesetz in Forschung und Lehre. Lund: Gleerup.
• Verner, Karl Adolf. 1876a. Eine Ausnahme der ersten Lautverschiebung. Kuhns Zeitschrift (Zeitschrift für vergleichende Sprachforschung) 23.97-130.
• Verner, Karl Adolf. 1876b. Zur Ablautsfrage. Kuhns Zeitschrift (Zeitschrift für vergleichende Sprachforschung) 21.131-38.
• Verner, Karl Adolf. 1903. Afhandlinger og breve udgivne af Selskab for germansk filologi. Copenhagen: Selskab for germansk filologi.