What is "verner's law"

Verner's law \Ver"ner's law\ (Philol.) A statement, propounded by the Danish philologist Karl Verner in 1875, which explains certain apparent exceptions to Grimm's law by the original position of the accent. Primitive Indo-European k, t, p, became first in Teutonic h, th, f, and appear without further change in old Teutonic, if the accent rested on the preceding syllable; but these sounds became voiced and produced g, d, b, if the accent was originally on a different syllable. Similarly s either remained unchanged, or it became z and later r. Example: Skt. sapt[=a] (accent on ultima), Gr. 'e`pta, Gothic sibun (seven). Examples in English are dead by the side of death, to rise and to rear.

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 the fricatives *β, *ð, *z, *ɣ, *ɣʷ respectively.In Proto-Germanic, voiced fricatives were allophones of their corresponding voiced plosives when they occurred between vowels, semivowels and liquids.

The situations where Verner's law applied resulted in fricatives in these very circumstances, so fricative can be used in this context.