Kluge's law
Kluge's law is a controversial Proto-Germanic sound law formulated by Friedrich Kluge. It purports to explain the origin of the Proto-Germanic long consonants *kk, *tt, and *pp (Proto-Indo-European lacked a phonemic length distinction for consonants) as originating in the assimilation of *n to a preceding voiced plosive consonant, under the condition that the *n was part of a suffix which was stressed in the ancestral Proto-Indo-European (PIE).