Grimm's law
sound shift in the Germanic languages in which PIE voiceless stops become fricatives, PIE voiced stops become voiceless and PIE voiced aspirated stops become voiced stops or fricatives / From Wikipedia, the free encyclopedia
Grimm's law is the name for sound laws in Proto-Indo-Germanic languages. In the first century BC some sounds in these languages changed. The law is named after the German linguist Jacob Grimm who described these changes in the early 19th century.
It is known that almost all modern European languages originated in (grew out of) early Indo-European language. Proto-Indo-European (PIE) is the reconstructed common ancestor of these modern languages.
Grimm's law is sometimes called first Germanic sound shift. There also was a second Germanic sound shift, which mostly affected German dialects, and happened much later.