Grimm's Law is a linguistic principle that describes how certain consonant sounds in the Proto-Indo-European language evolved into different sounds in the Germanic languages. It was first formulated by the linguist Jakob Grimm in the 19th century. The law outlines systematic sound changes, such as the transformation of the voiceless stops /p/, /t/, and /k/ into the voiceless fricatives /f/, /θ/ (th), and /h/, respectively. This law helps explain the phonetic differences between Germanic languages, like English and German, and their Indo-European relatives, such as Latin and Greek. For example, the Latin word "pater" (father) corresponds to the English "father," illustrating how the initial sound changed according to Grimm's Law.