Page "Grammatischer Wechsel" Paragraph 1

According to Grimm's law, the Proto-Indo-European ( PIE ) voiceless stops * p, * t, * k and * kʷ usually became Proto-Germanic * f, * θ ( dental fricative ), * x and * xʷ ( velar fricative ).

Karl Verner identified the principle that these instead become the voiced consonants * b, * d, * g, * gʷ if they were word-internal and immediately preceded by an unaccented vowel in PIE.

Furthermore, PIE * s, which usually came into Germanic unchanged, became * z in this position ; Proto-Germanic * z later became North-and West Germanic * r.