Grimm's law (also known as the First Germanic Sound Shift or the Rask's rule), named after Jacob Grimm, is a set of statements describing the inherited Proto-Indo-European (PIE) stop consonants as they developed in Proto-Germanic (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). 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 stops or fricatives (as allophones).
The chain shift can be abstractly represented as:
- bʰ → b → p → ɸ
- dʰ → d → t → θ
- gʰ → g → k → x
- gʷʰ → gʷ → kʷ → xʷ
Here each sound moves one position to the right to take on its new sound value. Notice that the new sound value entails the loss of a feature at each of the three steps. First the aspiration feature is lost, then voice and finally stop leaving a continuant.
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 correspondences between Latin p and Germanic f was first noted by Friedrich von Schlegel in 1806. In 1818 Rasmus Christian Rask elaborated the set of correspondences to include other Indo-European languages such as Sanskrit and Greek, and the full range of consonants involved. In 1822 Jacob Grimm, the elder of the Brothers Grimm, in his book Deutsche Grammatik, formulated the law as a general rule (and extended to include standard German).
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.
|Non-Germanic (unshifted) cognates||Change||Germanic (shifted) examples|
|Ancient Greek: πούς (poús), Latin: pēs, pedis, Sanskrit: pāda, Russian: под (pod) "under; floor", Lithuanian: pėda, Latvian pēda||*p→f [ɸ]||English: foot, West Frisian: foet, German: Fuß, Gothic: fōtus, Icelandic, Faroese: fótur, Danish: fod, Norwegian, Swedish: fot|
|Ancient Greek: τρίτος (tritos), Latin: tertius, Welsh: trydydd, Sanskrit: treta, Russian: третий (tretij), Lithuanian: trečias, Albanian: tretë||*t→þ [θ]||English: third, Old Frisian: thredda, Old Saxon: thriddio, Gothic: þridja, Icelandic: þriðji|
|Ancient Greek: κύων (kýōn), Latin: canis, Welsh: ci (pl. cwn)||*k→h [x]||English: hound, Dutch: hond, German: Hund, Gothic: hunds, Icelandic, Faroese: hundur, Danish, Norwegian, Swedish: hund|
|Latin: quod, Irish: cad, Sanskrit: kád, Russian: ко- (ko-), Lithuanian: kas||*kʷ→hw [xʷ]||English: what, Gothic: ƕa ("hwa"), Icelandic: hvað, Faroese: hvat, Danish: hvad, Norwegian: hva|
|Latin: verber "rod", Homeric Greek: ῥάβδος (rabdos) "rod, wand", Lithuanian: virbas||*b→p [p]||English: warp, West Frisian: werpe, Dutch: werpen, Icelandic: verpa, varpa, Faroese: verpa, Gothic wairpan|
|Latin: decem, Greek: δέκα (déka), Irish: deich, Sanskrit: daśan, Russian: десять (desyat'), Lithuanian: dešimt||*d→t [t]||English: ten, Dutch: tien, Gothic: taíhun, Icelandic: tíu, Faroese: tíggju, Danish, Norwegian: ti, Swedish: tio|
|Latin: gelū, Greek: γελανδρός (gelandrós), Lithuanian: gelmenis, gelumà||*g→k [k]||English: cold, West Frisian: kâld, Dutch: koud, German: kalt, Icelandic, Faroese: kaldur, Danish: kold, Norwegian: kald, Swedish: kall|
|Lithuanian: gyvas||*gʷ→kw [kʷ]||English: quick, West Frisian: kwik, kwyk, Dutch: kwiek, Gothic: qius, Icelandic, Faroese: kvikur, Danish: kvik, Swedish: kvick, Norwegian kvikk|
|Sanskrit: bhrātṛ||*bʰ→b [b]/[β]||English: brother, West Frisian, Dutch: broeder, German: Bruder, Gothic: broþar, Icelandic, Faroese: bróðir, Danish, Swedish, Norwegian: broder|
|Sanskrit: mádhu 'honey', Homeric Greek: μέθυ methu||*dʰ→d [d]/[ð]||English: mead, East Frisian: meede, Dutch: mede, Danish/Norwegian: mjød, Icelandic: mjöður , Swedish: mjöd|
|Ancient Greek: χήν (khēn), Sanskrit: hamsa (swan)||*gʰ→g [ɡ]/[ɣ]||English: goose, West Frisian: goes, guos, Dutch: gans, German: Gans, Icelandic: gæs, Faroese: gás, Danish, Norwegian, Swedish: gås|
|Homeric Greek: ἐάφθη (eáphthē) "sang, sounded", ὀμφή (omphē) "voice"||*gʷʰ→gw [ɡʷ]
|English: sing, West Frisian: sjonge, Dutch: zingen, German: singen, Gothic: siggwan, Old Icelandic: syngva, syngja, Icelandic, Faroese: syngja, Swedish: sjunga, Danish: synge/sjunge|
|Sanskrit: gharmá-, Avestan: garəmó, Old Prussian: gorme||*gʷʰ→gw→b, g or w
(Otherwise merged with existing g and w)
|English: warm, West Frisian: waarm, Dutch, German: warm, Swedish: varm, Icelandic: varmur|
- Note: Proto-Germanic *gʷ from Proto-Indo-European *gʷʰ has undergone further changes of various sorts. After *n it was preserved as *gʷ, but later changed to *g in West Germanic. Following vowels, it seems to have become *w, presumably through a fricative stage *ɣʷ. Word-initially, the most plausible reflex is a labiovelar stop *gʷ at first, but the further development is unclear. In that position, it became either *w, *g or *b during late Proto-Germanic. The regular reflex before *u would likely have been *g, due to loss of the labial element before a labial vowel. Perhaps the usual reflex was *b (as suggested by the connection of bid < *bidjana- and Old Irish guidid), but *w appears in certain cases (possibly through dissimilation when another labial consonant followed?), such as in warm and wife (provided that the proposed explanations are correct). Apparently, Proto-Germanic *hʷ voiced by Verner's law fell together with this sound and developed identically, compare the words for 'she-wolf': from Middle High German wülbe and Old Norse ylgr, one can reconstruct Proto-Germanic nominative singular *wulbī, genitive singular *wulgijōz, from earlier *wulgʷī, *wulgʷijōz.
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.
There are three main systematic exceptions.
1. The voiceless stops did not become fricatives if they were preceded by *s (itself a fricative).
|Non-Germanic examples||Change||Germanic examples|
|Latin: spuere, Lithuanian: spjáuti||*sp||English: spew, West Frisian: spije, Dutch: spuwen, German: speien, Danish, Norwegian, Swedish: spy, Icelandic: spýja, Faroese: spýggja, Gothic: speiwan|
|Latin: stāre, Irish: stad, Sanskrit: sta, Russian: стать (stat'), Lithuanian: stoti, Persian: ايستادن (istâdan)||*st||English: stand, Icelandic, Faroese, Norwegian: standa, Gothic: standan; West Frisian: stean, Dutch: staan, German: stehen, Danish, Swedish: stå|
|Lithuanian: skurdus||*sk||English: short, Old High German: scurz, Icelandic: skorta|
|Irish: scéal||*skʷ||English: scold, Icelandic: skáld, Norwegian: skald; West Frisian: skelle, Dutch: schelden, German: schelten|
2. The voiceless stop *t did not become a fricative if preceded by another stop, but the preceding stop was generally devoiced and then fricativised. This also happened to stops before *s, but that sound was not affected by Grimm's law.
Combined with the previous exception it is therefore most convenient to say that in a series of two obstruents, the second does not become a fricative but the first does. This is sometimes treated separately under the heading Germanic spirant law:
|Non-Germanic examples||Change||Germanic examples|
|Ancient Greek: κλέπτης (kleptēs), Old Prussian: au-klipts "hidden"||*pt→ft||Gothic: hliftus "thief"|
|Latin: atta, Greek: ἄττα (átta)||*tt→tt||Old High German: atto, Gothic: atta "father"|
|Ancient Greek: οκτώ (oktō), Irish: ocht, Latin: octō||*kt→ht||English: eight, West Frisian, Dutch, German: acht, Gothic: ahtáu, Icelandic: átta (pronounced [ˈauhta])|
|Irish: anocht, Latin: nox, noct-, Greek: νύξ, νυκτ- (núks, nukt-), Sanskrit: नक्तम् (naktam), Lithuanian: naktis, Hittite (genitive): nekuz (pronounced /nekʷts/)||*kʷt→ht||English: night, West Frisian, Dutch, German: nacht, Gothic: nahts, Icelandic: nótt (pronounced [ˈnouht])|
- Note: Icelandic nótt comes from Proto-Germanic *naht-, with the /ht/ regularly becoming /tt/, which was originally pronounced [tː] 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.
3. 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). (This is not necessarily an actual exception: the traditional dating of Verner's Law occurring after Grimm's would mean that the consonants affected did undergo Grimm's law, and were only changed later.)
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).
See also 
- 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.
- Armenian, another Indo-European language, has experienced a similar evolution.
- Campbell, Lyle (2004). Historical linguistics (2nd ed. ed.). Cambridge: MIT Press. p. 49. ISBN 0-262-53267-0.
- Kuiper, F B J (1995). "Gothic 'bagms' and Old Icelandic 'ylgr'". North-Western European Language Evolution (NOWELE) (25): 63–88.