Grimm's law
A seminal insight into how the Germanic languages diverged from their Indo-European ancestor had been established in the early nineteenth century, and had been formulated as Grimm's law. Amongst other things, Grimm's law described how the Proto-Indo-European voiceless stops ', ', "", and regularly changed into Proto-Germanic (bilabial fricative ), (dental fricative ), (velar fricative ), and (velar fricative ).