Verner's Law

Verner's law, stated by Karl Verner in 1875, describes a historical sound change in the Proto-Germanic language whereby voiceless fricatives *f, *þ, *s, *h, *hʷ, when immediately following an unstressed syllable in the same word, underwent voicing and became respectively the fricatives *b, *d, *z, *g, *gʷ.

(In Proto-Germanic, voiced fricatives * were allophones of their corresponding voiced plosives * when they occurred between vowels, semivowels and liquids, so we write them here as *b, *d, *g. But the situations where Verner's law applied resulted in fricatives in these very circumstances, so we understand these phonemes as fricatives in this context.)