What Grimm's Law describes
Grimm's Law is a set of regular sound changes that distinguished the Germanic languages from the rest of the Indo-European family. Formulated by Jacob Grimm in 1822, building on earlier observations by Rasmus Rask, it describes how Proto-Indo-European stop consonants shifted systematically into Proto-Germanic. The change came in three linked moves: voiceless stops became voiceless fricatives (p to f, t to th, k to h), voiced stops became voiceless stops (b to p, d to t, g to k), and voiced aspirated stops became plain voiced stops.
These shifts are why Latin pater corresponds to English father, Latin tres to English three, and Latin cornu to English horn.
Why it matters in linguistics
Grimm's Law was one of the first demonstrations that sound change is regular and law-like rather than random, a cornerstone of the comparative method. Apparent exceptions were later explained by Verner's Law, which showed the outcome depended on the position of the original Indo-European accent. Together they let linguists reconstruct ancestral forms with confidence and trace which languages belong to the Germanic branch.