Grimm's law

Grimm's law is the name for sound laws in Proto-Indo-Germanic languages. In the first century BC some sounds in these languages changed. The law is named after the German linguist Jacob Grimm who described these changes in the early 19th century.
It is known that almost all modern European languages originated in (grew out of) early Indo-European language. Proto-Indo-European (PIE) is the reconstructed common ancestor of these modern languages.
Grimm's law is sometimes called first Germanic sound shift. There also was a second Germanic sound shift, which mostly affected German dialects, and happened much later.
Historical background
Jacob Grimm based his theory on Danish philologist Rasmus Christian Rask's essay On the Origin of the Icelandic Language. He examined the relationship between consonants in Germanic languages and other Indo-European languages such as Latin or French. The results of the change in consonants are still visible in English, Dutch, and the Scandinavian languages.
The first Germanic sound shift
A chain shift is a series of interrelated sound changes. The assumption behind the chain shift is that all the phonemes of a language build a balanced system so that a change in one part of the system can cause changes in its other parts.
Many chain shifts are vowel shifts, because many sets of vowels are naturally arranged on a multi-value scale (e.g. vowel height or frontness). However, chain shifts can also occur in consonants. The modern word apple used to be pronounced with b (Gaulish abalom or Lithuanian obuolys), a change that occurred with Grimm's law.
Therefore, in terms of consonants, the shift from PIE to Proto-Germanic (PG) can be split into three stages:
PIE voiceless stop consonants (stops) changing into voiceless fricative consonants (fricatives). The voiceless stops remain unaffected after the letter s.
PIE unaspirated voiced stops (aspirated consonants) changing into unaspirated voiceless stops.
PIE aspirated voiced stops changing into unaspirated voiced fricatives and/or unaspirated voiced stops (allophones at the time).
Explanation
The first group of voiceless stop consonants:
For example, the p-sound changed to an f-sound. Latin pater became father in modern English. Equally, duo became two.
Additionally, the PIE treyes became thriz in Proto-Germanic which turned into the modern English three (as in ['thri:]).
The word kaput or more known Latin equivalent corpus shifted towards head.
Lastly, PIE *kwo- transformed into Old English hwaet.
All the words that consisted of the presented consonants were affected by Grimm's law. Some words may have only undergone one or two of the changes, while others may have undergone all of them.
Similarities in Germanic languages
The impact of the first Germanic sound shift can be followed across most of the Germanic languages. We can see that many words are, due to the shift, similar (in sound and meaning). Grimm's law provides an insight into why and how core words sound so similar in Germanic languages, yet are in some respects quite distinct from other Indo-European languages:
bh changed into b (English: brother; West Frisian, Dutch: broeder; German: Bruder; Gothic: brothar; Icelandic, Faroese: brodir; Danish, Norwegian, Swedish: broder)
g switched to k (English: cold; West Frisian: kald; Dutch: koud; German: kalt; Icelandic, Faroese: kaldur; Danish: kold; Norwegian: kald; Swedish: kall)
These changes also could have created consonant clusters, as in words which contain two or more consonants:
*t with the shift *kt --> *ht (English: eight; West Frisian, Dutch, German: acht; Gothic: ahtau; Icelandic: atta)
and *k with the shift *kwt-ht (English: night; West Frisian, Dutch, German: Nacht; Gothic: nahts; Icelandic: nott)
When did it happen?
It is not possible to exactly say, when it happened. Most people say that this was around 500 BC. This is because that are several loanwords, in Germanic languages, which do not occur before the 5th century BC. There are some examples, that the shift completed in the 1st century BC.
'cimbri teutonique' - should have been 'chimbri theudonique' after the shift.
Caesar talks about 'Vacalus' (the River Waal, one of the two main branches of the estuary of the Rhine). About 150 years later, Tacitus uses Vahalis for the same river.
In De Bello Gallico, Cesar mentions tribes around the river Meuse: "Condruses, Eburones, Caerosos, Paemanos, qui uno nomine Germani appellantur" (Condrusi, Eburones,Caerosi and Paemani, which are called Germani by name) - (Bello Gallico, 2,4,10). Today, most people think that Eburones is the only Germanic name (with shift); the others are Celtic (without shift).