A further important step was taken when in 1822 a German philologist, Jacob Grimm, following up a suggestion of a Danish contemporary, Rasmus Rask, formulated an explanation that systematically accounted for the correspondences between certain consonants in the Germanic languages and those found for example in Sanskrit, Greek, and Latin. His explanation, although subsequently modified and in some of the details of its operation still a subject of dispute, is easily illustrated. According to Grimm, a p in Indo-European, preserved as such in Latin and Greek, was changed to an f in the Germanic languages. Thus we should look for the English equivalent of Latin piscis or pēs to begin with an f, and this is what we actually find, in fish and foot respectively. What is true of p is true also of t and k: in other words, the original voiceless stops (p, t, k) were changed to fricatives (f, þ, h). So Latin trēs=English three, Latin centum=English hundred. A similar correspondence can be shown for certain other groups of consonants, and the Consequently Sanskrit bhárāmi (Greek )=English bear, Sanskrit dhā=English do, Latin hostis (from *ghostis)=English guest. And the original voiced stops (b, d, g) changed to voiceless ones in the Germanic languages, so that Latin cannabis=English hemp (showing also the shift of initial k to h), Latin decem=English ten, Latin genu=English knee. In High German some of these consonants underwent a further change, known as the Second or High German Sound-Shift. It accounts for such differences as we see in English open and German offen, English eat and German essen.formulation of these correspondences is known as Grimm’s Law. The cause of the change is not known. It must have taken place sometime after the segregation of the Germanic from neighboring dialects of the parent language. There are words in Finnish borrowed from Germanic that do not show the change and that therefore must have resulted from a contact between Germanic and Finnish before the change occurred. There is also evidence that the shifting was still occurring as late as about the fifth century B.C. It is often assumed that the change was due to contact with a non-Germanic population. The contact could have resulted from the migration of the Germanic tribes or from the penetration of a foreign population into Germanic territory. Whatever its cause, the Germanic sound-shift is the most distinctive feature marking off the Germanic languages from the languages to which they are related.
Certain apparent exceptions to Grimm’s Law were subsequently explained by Karl Verner and others. It was noted that between such a pair of words as Latin centum and English hundred the correspondence between the c and h was according to rule, but that between the t and d was not. The d in the English word should have been a voiceless fricative, that is, a þ. In 1875 Verner showed that when the Indo-European accent was not on the vowel immediately preceding, such voiceless fricatives became voiced in Germanic. In West Germanic the resulting ð became a d, and the word hundred is therefore quite regular in its correspondence with centum. The explanation was of importance in accounting for the forms of the preterite tense in many strong verbs. Thus in Old English the preterite singular of cweþan (to say) is ic cwœþ but the plural is we In the latter word the accent was originally on the ending, as it was in the past participle (cweden), where we also have a d.
The formulation of this explanation is known as Verner’s Law, and it was of great significance in vindicating the claim of regularity for the sound-changes that Grimm’s Law had attempted to define.