Grimm's Law

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A formula describing the regular changes undergone by Indo-European stop consonants represented in Germanic, essentially stating that Indo-European p, t, and k became Germanic f, th, and h; Indo-European b, d, and g became Germanic p, t, and k; and Indo-European bh, dh, and gh became Germanic b, d, and g.