Grimm's law

Grimm's law (also known as the First Germanic Sound Shift) is a set of sound laws describing the Proto-Indo-European (PIE) stop consonants as they developed in Proto-Germanic in the 1st millennium BC. First discovered by Rasmus Rask but systematically put forward by Jacob Grimm, it establishes a set of regular correspondences between early Germanic stops and fricatives and stop consonants of certain other centum Indo-European languages.