Grimm's law

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Grimm's law, also known as the First Germanic Sound Shift or Rask's rule, is a set of sound laws describing the Proto-Indo-European (PIE) stop consonants as they developed in Proto-Germanic in the first 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 Indo-European languages.