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What is Grimm’s law and how did it change Germanic consonants?

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Thematic map of Europe displaying the distribution of Germanic languages, with different colors indicating regions where English, German, Netherlandic (Dutch), Frisian, Swedish, Danish, Norwegian, Icelandic, and Faroese are spoken. Striped areas mark regions where bilingualism is common, and language regions cover countries such as the United Kingdom, Germany, the Netherlands, Denmark, Sweden, Norway, Iceland, and parts of surrounding countries. — Germanic languages Distribution of the Germanic languages in Europe.

Grimm’s law is a foundational principle in historical linguistics that describes the regular, systematic shifts in consonant sounds that distinguish the Germanic languages from other Indo-European languages. Formulated by Jacob Grimm, the law demonstrates that sound change is a predictable process rather than a random occurrence.1

The law identifies three primary shifts that occurred as Proto-Indo-European evolved into Proto-Germanic:2

Voiceless stops to voiceless fricatives: Ancient voiceless stops (p, t, k) shifted to become voiceless fricatives (f, th, h).12
Voiced stops to voiceless stops: Ancient voiced stops (b, d, g) shifted to become voiceless stops (p, t, k).12
Voiced aspirated stops to voiced fricatives: Ancient voiced aspirated stops (bh, dh, gh) shifted to become voiced fricatives (or voiced stops in some contexts, such as b, d, g in English).12

These shifts explain consistent correlations between words across languages, such as the relationship between Latin duo and English two, or Greek pyr and English fire.12