Grimm's law

Named for Jacob Grimm (1785–1863), German author, linguist and folklorist.

1(Indo-European studies) A sound change that affected the Proto-Indo-European stop consonants in the development of Proto-Germanic, causing devoicing or change into a fricative.
2(Indo-European studies) The description of this sound change (as originally formulated by Jacob Grimm), which establishes a set of regular correspondences between early Germanic stops and fricatives, and the consonants of Proto-Indo-European and most other Indo-European languages.