In linguistic circles Jacob Grimm (1785-1863) is better known for his discovery of phonetic changes in the development of Proto-Germanic than for his German folk tales.
Jacob was trained as a lawyer but was more interested in history and language – early German literature in particular. He authored many scholarly books including Geschichte der Deutschen Sprache (History of the German Language), Deutsche Grammatik (German Grammar), and the monumental Deutsches Wörterbuch (German Dictionary). In 1812 and 1814 he and his younger brother Wilhelm published their collection of old German folk tales as Grimms Märchen (Grimms’ Fairy Tales) Volumes 1 and 2.
Grimm’s Law was first published in the second edition of his German Grammar in 1822. It is considered significant in linguistic science for introducing “…a rigorous methodology to historic linguistic research.” [http://en.wikipedia.org/wiki/Jacob_Grimm].
Sometime during the progression from Proto-Indo-European (about 2000 BC) to Proto-Germanic (about 500 BC), certain sound shifts in consonants occurred which were nearly universal (occurring to all words). These changes did not occur in any other PIE daughter languages such as the Proto-Romance language, so are observable with comparisons between English words taken directly from Latin, French or other Romance languages and those from its Germanic roots.
Greatly simplified, the changes are (with examples):
Labials (sounds made with the lips)
p changes to f: ped / foot; pater / father
f changes to b: fund / bottom
b changes to p: labial / lip
Velars (sounds made mid-palate)
k changes to h: canine / hound
h changes to g: host / guest
g changes to k: genuflect / knee
Dentals (sounds made with the tongue and teeth)
t changes to th: triple / three
th changes to d: thyroid / door
d changes to t: duo / two
Note that in all three groups, the changes go full circle, sort of like musical chairs.
Verner’s Law, developed by Danish linguist Karl Verner, explains further changes that occurred later to certain consonants in certain conditions (depending on the accent of the preceding syllable).
These sound changes are evident in all of the modern Germanic languages: German, Frisian, Dutch (and Africaans), Danish, Swedish, Norwegian, Icelandic, Faroese, Luxembourgish, and of course English.
Sound changes like this are common in language history and take several generations, often a few centuries, to complete. A more recent example in English is known as the Great Vowel Shift which occurred between 1400 and 1600 (between Chaucer and Shakespeare). That’s a subject for a future post.