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 "obuolỹs"), 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:
Explanation.
The first group of voiceless stop consonants:
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:
These changes also could have created consonant clusters, as in words which contain two or more consonants:
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.