The Problem
When Grimm's law was discovered, a strange irregularity was spotted in its operation. The Proto-Indo-European (PIE) voiceless stops *p, *t and *k should have changed into Proto-Germanic (PGmc) *f (bilabial fricative ), *þ (dental fricative ) and *h (velar fricative ), according to Grimm's Law. Indeed, that was known to be the usual development. However, there appeared to be a large set of words in which the agreement of Latin, Greek, Sanskrit, Baltic, Slavic etc. guaranteed PIE *p, *t or *k, and yet the Germanic reflex was voiced (*b, *d or *g).
At first, irregularities did not cause concern for scholars since there were many examples of the regular outcome. Increasingly, however, it became the ambition of linguists to formulate general and exceptionless rules of sound change that would account for all the data (or as close to all the data as possible), not merely for a well-behaved subset of it (see Neogrammarians).
One classic example of PIE *t → PGmc *d is the word for 'father'. PIE *ph2tēr (here, the macron marks vowel length) → PGmc *fadēr (instead of expected *faþēr). The structurally similar family term *bʱrātēr 'brother' did indeed develop as predicted by Grimm's Law (Gmc. *brōþēr). Even more curiously, we often find both *þ and *d as reflexes of PIE *t in different forms of one and the same root, e.g. *werþanan 'to turn', preterite singular *warþ 'he turned', but preterite plural and past participle *wurd- (plus appropriate inflections).
Read more about this topic: Verner's Law
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