Grimm's Law — the first Germanic sound shift

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The set of consonant changes that separates the Germanic languages from the rest of Indo-European, and the first sound change ever stated as a system rather than a list. Inherited voiceless stops became voiceless fricatives — p, t, k to f, th, h; inherited voiced stops became voiceless — b, d, g to p, t, k; and the inherited breathy-voiced series became plain voiced stops. Its effects are visible in any pair of English and Latin words descended from the same root: Latin pater against English father, Latin decem against English ten, Latin cornu against English horn, Latin genus against English kin. The shift was described by the Danish scholar Rasmus Rask before Jacob Grimm, in work Grimm himself acknowledged; the law carries Grimm's name because his 1822 formulation in the Deutsche Grammatik presented it as a connected chain in which each series moved into the position vacated by another. That framing is what made it a law rather than a collection — it implied that changes were structured, and therefore that they could be predicted and violated.

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