See M. Tatar, ed., The Annotated Brothers Grimm (2004); L. Segal and M. Sendak, ed. The Juniper Tree and Other Tales from Grimm (1973); J. Zipes, The Brothers Grimm (1989).
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.
|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, Welsh pel|
|*t→þ||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→x (x later became h)||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||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, Icelandic, Faroese: kvikur, Swedish: kvick, Norwegian kvikk||Lithuanian: gyvas|
|*bʰ→b||English: brother, Dutch: broeder, German: Bruder, Gothic: broþar, Icelandic, Faroese: bróðir, Danish, Swedish: broder, Norwegian bror||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)|
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-.
|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, Norwegian, 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||Irish: scioll|
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)|
|*kt→ht||English: eight, Dutch: acht, Frisian: acht, German: acht, Gothic: ahtáu Icelandic: átta ()||Ancient Greek: οκτώ (oktō), Latin: octō|
|*kʷt→h(w)t||English: night, Old High German: naht, Old Frisian, Dutch, German: nacht, Gothic: nahts, Icelandic: nótt ()||Greek: nuks, nukt-, Latin: nox, noct-, Sanskrit: naktam, Lithuanian: naktis|