Grimm's Law
Definition - A set of sound laws that describes the systematic changes experienced by Indo-European stop consonants as the language evolved into Proto-Germanic and other Indo-European languages.

In a nutshell, it says that:
(1) The Indo-European consonants p, t, and k evolved into the Proto-Germanic consonants f, th, and h.
(2) The Indo-European consonants b, d, and g evolved into the Proto-Germanic p, t, and k.
(3) The Indo-European bh, dh, and gh evolved into the Proto-Germanic b, d, and g.

Example -
According to Grimm's Law, the Latin or Greek f corresponds to the Germanic b, thus the Latin flos, flower, is related to the English word blossom.

Etymology -
The phenomenon was first described in 1818 by Rasmus Rask, but it was set out in detail by Jacob Grimm in 1822.

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