Grimm's law (also known as the [First] Germanic Sound Shift) was the first non-trivial systematic sound change ever to be discovered; its formulation was a turning-point in the development of linguistics, enabling the introduction of rigorous methodology in historical linguistic research. The "law" was discovered about 1820 by Jakob Grimm, the younger of the Brothers Grimm. It establishes a set of regular correspondences between early Germanic stops and fricatives (see: Consonant) and the stop consonants of certain other Indo-European languages (Grimm used mostly Latin and Greek for illustration). As formulated nowadays, Grimm's Law describes the development of inherited Proto-Indo-European (PIE) stops in Proto-Germanic (PGmc, the common ancestor of the Germanic branch of the Indo-European family). It consists of three parts (the presentation below is simplified for the sake of clarity):

• Proto-Indo-European voiceless stops change into voiceless fricatives:

PIE *p, *t, *k > PGmc *f, *þ, *x

Note 1: *þ stands for "th" as in thick, *x for Scots "ch" as in loch)

Note 2: Here, and in the other parts of Grimm's Law, the place of articulation remains roughly the same, so that a labial stop (*p) becomes a labial fricative (*f), etc.

• Proto-Indo-European voiced stops become voiceless:

PIE *b, *d, *g > PGmc *p, *t, *k

• Proto-Indo-European voiced aspirated stops lose their aspiration and change into plain voiced stops:

PIE *b^h, *d^h, *g^h > PGmc *b, *d, *g

For example:

PIE *petro- > PGmc *feþra- (English feather)

PIE *tnwi- 'thin' > PGmc. *þunni-

PIE *ed- 'eat' > PGmc. *et-

PIE *sed- 'sit' > PGmc. *set-

PIE *wodr- 'water' > PGmc *watr-

PIE *med^hu- 'mead' > PGmc *medu-

PIE *b^her- 'carry, bear' > PGmc *ber-

PIE *g^{^}eus- 'taste' > PGmc *kius- 'choose'

PIE *g^hord^ho- 'enclosed place' > PGmc *gard-

PIE *leg^h- 'lie (recline)' > PGmc *leg-

There are some subtle complications, ignored here (e.g. voiceless stops are exempted from the change if preceded by *s). They were either accounted for by Grimm himself or patiently sorted out by later scholars. The most recalcitrant set of apparent exceptions to Grimm's Law, which defied linguists for a few decades, eventually received a brilliant explanation from the Danish linguist Karl Verner (see the article on Verner's law for details).

The Germanic "sound laws", combined with regular changes reconstructed for other Indo-European languages, allow one to define the expected sound correspondences between different branches of the family.

For example, Germanic (word-initial) *b- corresponds regularly to Latin *f-, Greek p^h-, Sanskrit b^h-, Slavic, Baltic or Celtic b-, etc., while Germanic *f- corresponds to Latin, Greek, Sanskrit, Slavic and Baltic p- and to zero (no initial consonant) in Celtic. The former set goes back to PIE *b^h- (faithfully reflected in Sanskrit and modified in various ways elsewhere), and the latter set to PIE *p- (shifted in Germanic, lost in Celtic, preserved in the other groups mentioned here).

---

Grimm also discovered another ("Second") consonant shift, which accounts for the consonant system of High German. It did not operate in the remaining Germanic languages, which meant that e.g. the English system of stops and fricatives is more archaic (closer to Proto-Germanic) than that of Modern German.

e.g.

English two vs German zwei /ts-/

English pipe vs German Pfeife /pf-f-/

English make vs German machen /-x-/