Grimm's law (also known as the First Germanic Sound Shift or the Rask's-Grimm's rule) named for Jacob Grimm, is a set of statements describing the inherited Proto-Indo-European (PIE) stops as they developed in Proto-Germanic (PGmc, the common ancestor of the Germanic branch of the Indo-European family) in the 1st millennium BC. It establishes a set of regular correspondences between early Germanic stops and fricatives and the stop consonants of certain other centum Indo-European languages (Grimm used mostly Latin and Greek for illustration). As it is presently formulated, Grimm's Law consists of three parts, which must be thought of as three consecutive phases in the sense of a chain shift:
- Proto-Indo-European voiceless stops change into voiceless fricatives.
- Proto-Indo-European voiced stops become voiceless stops.
- Proto-Indo-European voiced aspirated stops become voiced fricatives; ultimately, in most Germanic languages these voiced fricatives become voiced stops.
The voiced aspirated stops may have first become voiced fricatives before hardening to the voiced unaspirated stops "b", "d", and "g" under certain conditions, however some linguists dispute this. See Proto-Germanic phonology.
Grimm's law was the first non-trivial systematic sound change to be discovered in linguistics; its formulation was a turning point in the development of linguistics, enabling the introduction of a rigorous methodology to historical linguistic research. The "law" was discovered by Friedrich von Schlegel in 1806 and Rasmus Christian Rask in 1818, and later elaborated (i.e. extended to include standard German) in 1822 by Jacob Grimm, the elder of the Brothers Grimm, in his book Deutsche Grammatik.
Further changes following Grimm's Law, as well as sound changes in other Indo-European languages, can sometimes obscure its effects. The most illustrative examples are used here.
|Change||Germanic (shifted) examples||Non-Germanic (unshifted) cognates|
|*p→f||English: foot, German: Fuß, Gothic: fōtus, Icelandic, Faroese: fótur, Danish: fod, Norwegian, Swedish: fot||Ancient Greek: πούς (pūs), Latin: pēs, pedis, Sanskrit: pāda, Russian: под (pod), Lithuanian: pėda|
|*t→þ [θ]||English: third, Old High German: thritto, Gothic: þridja, Icelandic: þriðji||Ancient Greek: τρίτος (tritos), Latin: tertius, Gaelic treas, Irish: trí, Sanskrit: treta, Russian: третий (tretij), Lithuanian: trečias|
|*k→h [x]||English: hound, Dutch: hond, German: Hund, Gothic: hunds, Icelandic, Faroese: hundur, Danish, Norwegian, Swedish: hund||Ancient Greek: κύων (kýōn), Latin: canis, Gaelic, Irish: cú, Welsh ci|
|*kʷ→hw [xw]||English: what, Gothic: ƕa ("hwa"), Danish hvad, Icelandic: hvað, Faroese hvat, Norwegian: hva||Latin: quod, Gaelic: ciod, Irish: cad, Sanskrit: ka-, kiṃ, Russian: ко- (ko-), Lithuanian: ką'|
|*b→p||English: warp; Swedish: värpa; Dutch: werpen; Icelandic, Faroese: varpa, Gothic wairpan||Latin: verber|
|*d→t||English: ten, Dutch: tien, Gothic: taíhun, Icelandic: tíu, Faroese: tíggju, Danish, Norwegian: ti, Swedish: tio||Latin: decem, Greek: δέκα (déka), Gaelic, Irish: deich, Sanskrit: daśan, Russian: десять (desyat'), Lithuanian: dešimt, Welsh deg|
|*g→k||English: cold, Dutch: koud, German: kalt, Icelandic, Faroese: kaldur, Danish: kold, Norwegian: kald, Swedish: kall,||Latin: gelū|
|*gʷ→kw||English: quick, Frisian: quick, queck, Dutch: kwiek, Gothic: qius, Old Norse: kvikr, Danish: kvik, Icelandic, Faroese: kvikur, Swedish: kvick, Norwegian kvikk||Lithuanian: gyvas|
|*bʰ→b||English: brother, Dutch: broeder, German: Bruder, Gothic: broþar, Icelandic, Faroese: bróðir, Danish, Swedish, Norwegian: broder||Sanskrit: (bhrātā), Russian: брат (brat), Lithuanian: brolis, Old Church Slavonic: братръ (bratru)|
|*dʰ→d||English: door, Frisian: doar, Dutch: deur, Gothic: daúr, Icelandic, Faroese: dyr, Danish, Norwegian: dør, Swedish: dörr||Irish: doras, Sanskrit: dwār, Russian: дверь (dver'), Lithuanian: durys|
|*gʰ→g||English: goose, Frisian: goes, Dutch: gans, German: Gans, Icelandic: gæs, Faroese: gás, Danish, Norwegian, Swedish: gås||Russian: гусь (gus')|
|*gʷʰ→gw→w||English: wife, Proto-Germanic: wiban (from former gwiban), Old Saxon, Old Frisian: wif, Dutch: wijf, Old High German: wib, German: Weib, Old Norse: vif, Icelandic: víf, Faroese: vív, Danish, Swedish, Norwegian: viv||Tocharian A: kip, B: kwípe (vulva)|
- Note: Some linguists dispute the origin of the word "wife". Calvert Watkins has assumed the root word is Proto-Indo-European *gʷʰíbʰ-. 
This is strikingly regular. Each phase involves one single change which applies equally to the labials (p, b, bʰ, f) and their equivalent dentals (t, d, dʰ, þ), velars (k, g, gʰ, h) and rounded velars (kʷ, gʷ, gʷʰ, hw). The first phase left the phoneme repertoire of the language without voiceless stops, the second phase filled this gap but created a new one, and so on until the chain had run its course.
Note: Icelandic hv has actually reverted Grimm's Law in the last few generations, and is now pronounced [kʰv] or [kʰf]. Cf. also nynorsk kv-/k-.
The voiceless stops did not become fricatives if they were preceded by *s (itself a fricative).
|Change||Germanic examples||Non-Germanic examples|
|*sp||English: spew, Gothic: speiwan, Dutch: spuien, German: speien, Danish, Norwegian, Swedish: spy, Icelandic: spýja, Faroese: spýggja||Latin: spuere|
|*st||English: stand, Dutch: staan, German: stehen, Icelandic, Faroese: standa, Danish, Swedish: stå||Latin: stāre, Irish: stad, Sanskrit: sta, Russian: стать (stat'), Lithuanian: stoti|
|*sk||English: short, Old Norse and Icelandic: skorta, Old High German: scurz||Lithuanian: skurdus|
|*skʷ||English: scold, Old Norse: skäld, Icelandic: skáld, Dutch: schelden, Norwegian: skald||Irish: scioll|
- Some linguists dispute the origin of the word "scold", but Julius Pokorny among others proposed *skwetlo is the assumed root.
- Dutch has *k → *h (ch) even before *s.
Furthermore, the voiceless stop *t also did not become a fricative if preceded by *p, *k, or *kʷ (themselves voiceless stops). The voiceless stop it was preceded by did fricativize, however. (In other words, at the time in history when voiceless stops fricativized in Proto-Germanic, that fricativization only affected leading voiceless stops when paired with the voiceless stop *t.) This is sometimes treated separately under the heading Germanic spirant law:
|Change||Germanic examples||Non-Germanic examples|
|*pt→ft||Gothic: hliftus "thief"||Ancient Greek: κλέπτης (kleptēs), Sanskrit: स्तेन (sten)|
|*kt→ht||English: eight, Dutch: acht, Frisian: acht, German: acht, Gothic: ahtáu, Icelandic: átta (Template:Pronounced)||Ancient Greek: οκτώ (oktō), Latin: octō, Sanskrit: अष्ट (aṣṭa),|
|*kʷt→h(w)t||English: night, Old High German: naht, Old Frisian, Dutch, German: nacht, Gothic: nahts, Icelandic: nótt (Template:Pronounced)||Greek: nuks, nukt-, Latin: nox, noct-, Sanskrit: नक्तम् (naktam), Lithuanian: naktis|
- Note: Icelandic nótt comes from Proto-Germanic *naht-, with the /ht/ regularly becoming /tt/, which was originally pronounced [t:] before pre-aspirating. Thus, the [h] of the modern Icelandic form is not a direct descendant of ancient /h/. The same ancestry holds for the /tt/ of Icelandic átta as well. 
The most recalcitrant set of apparent exceptions to Grimm's Law, which defied linguists for a few decades, eventually received explanation from the Danish linguist Karl Verner (see the article on Verner's law for details).
Correspondences to PIE
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 pʰ-, Sanskrit bʰ-, 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 *bʰ- (faithfully reflected in Sanskrit and modified in various ways elsewhere), and the latter set to PIE *p- (shifted in Germanic, lost in Celtic, but preserved in the other groups mentioned here).
- Verner's law
- High German consonant shift
- Glottalic theory
- The Tuscan gorgia, a similar evolution differentiating the Tuscan dialects from Standard Italian.
- The Uralic Hungarian language was also affected by a similar process, leading to a high frequency of f and h, and can be compared to Finnish, which did not change this way.