A seminal insight into how the Germanic languages diverged from their Indo-European ancestor had been established in the early nineteenth century, and had been formulated as Grimm's law.
guaranteed Proto-Indo-European *p, *t or *k, and yet the Germanic reflex was not the expected, unvoiced fricatives *f, *þ, *h, *hʷ but rather their voiced counterparts *β, *ð, *ɣ, *ɣʷ.
[2] In the structurally similar family term *bʰréh₂tēr 'brother', Proto-Indo-European *t did indeed develop as predicted by Grimm's Law (Germanic *brōþēr).
[3] Even more curiously, scholars often found both *þ and *ð as reflexes of Proto-Indo-European *t in different forms of one and the same root, e.g. *werþaną 'to turn', preterite third-person singular *warþ 'he turned', but preterite third-person plural *wurðun and past participle *wurðanaz.
This is usually thought to be because Gothic eliminated most Verner's law variants through analogy with the unaffected consonants.
[5] Karl Verner published his discovery in the article "Eine Ausnahme der ersten Lautverschiebung" (An exception to the first sound shift) in Kuhn's Journal of Comparative Linguistic Research in 1877,[6] but he had already presented his theory on 1 May 1875 in a comprehensive personal letter to his friend and mentor, Vilhelm Thomsen.
[7] Verner's theory was received with great enthusiasm by the young generation of comparative philologists, the so-called Junggrammatiker, because it was an important argument in favour of the Neogrammarian dogma that the sound laws were without exceptions ("die Ausnahmslosigkeit der Lautgesetze").
However, the syllabic stress shift erased the conditioning environment, and made the variation between voiceless fricatives and their voiced alternants look mysteriously haphazard.
[8] But it has been pointed out that, even if the sequence is reversed, the result can be just the same given certain conditions, and the thesis that Verner's Law might have been valid before Grimm's Law—maybe long before it—has been finding more and more acceptance.
The usual proposed explanation for this is to postulate aspiration in the voiceless stops of the dialect of Indo-European that gave rise to Proto-Germanic.