The first major sound change law discovered. It described the shift of stops which occured during the change from Proto-Indo-European to Proto-Germanic. It was first postulated by Friedrich von Schegel in 1806, then by Rasmus Christian Rask in 1818, and finally enunciated in its most famous form by Jakob Grimm in 1822. It is composed of three parts:
1. The shift of voiceless stops to voiceless fricatives, thus p > f, t > th, and k > x.
2. The shift of voiced stops to voiceless stops, thus b > p, d > t, and g > k.
3. The deaspiration of the aspirated stops, thus bh > b, dh > d, and gh > g.
Some exceptions to Grimm's Law exist, most notably those stated by Karl Verner in Verner's law.
Thus through Grimm's Law, PIE petro > PG fethra, PIE wodr > PG watr, ghordho > gard, etc.