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.