How did Jacob Grimm contribute to the study of Indo-European languages?

Jacob Grimm was a foundational figure in historical linguistics whose work established scientific methods for studying language relationships and development.1

His most significant contribution is known as Grimm’s law, a principle demonstrating the regularity of sound correspondences among consonants in genetically related languages. By showing that Proto-Indo-European stops shifted into Proto-Germanic fricatives and stops in a predictable, systematic manner, he provided a rigorous basis for comparative linguistics.123

Grimm further advanced the field through his extensive work, Deutsche Grammatik (1819–37). In this study, he traced the historical development of Germanic languages and represented the natural laws of sound change for both vowels and consonants. His methodology influenced the study of Romance and Slavic languages as well, helping to solidify the comparative method used to reconstruct Proto-Indo-European.13