Grimm's law

the statement of the regular pattern of consonant correspondences presumed to represent changes from Proto-Indo-European to Germanic, according to which voiced aspirated stops became voiced obstruents, voiced unaspirated stops became unvoiced stops, and unvoiced stops became unvoiced fricatives: first formulated in 1820–22 by Jakob Grimm, though the facts had been noted earlier by Rasmus Rask.