Verner's law

Verner's law describes a historical sound change in the Proto-Germanic language whereby consonants that would usually have been the voiceless fricatives *f, *þ, *s, *h, *hʷ, following an unstressed syllable, became the voiced fricatives *β, *ð, *z, *ɣ, *ɣʷ. The law was formulated by Karl Verner, and first published in 1877.