How did 19th-century linguists demonstrate that Indo-European languages were related?

Nineteenth-century linguists demonstrated the relationship between Indo-European languages by developing the comparative method, which reconstructs ancestral languages by analyzing systematic correspondences in vocabulary and grammar across related languages.12

This process was catalyzed by Sir William Jones, who in 1786 observed that Sanskrit shared such strong affinities in verb roots and grammatical forms with Greek and Latin that they must have descended from a common source.34

Key developments included:
* Morphological Comparison: In 1816, Franz Bopp demonstrated relationships through detailed analysis of verb structures.4
* Systematic Sound Laws: Rasmus Rask and Jacob Grimm identified regular sound correspondences, most notably Grimm’s law, which explained how consonants shifted predictably between languages.34
* Neogrammarian Principle: By the 1870s, linguists established that sound changes are regular and exceptionless, allowing for the rigorous reconstruction of Proto-Indo-European.14

These methods allowed scholars to classify diverse groups—including Slavic, Baltic, Celtic, and Germanic languages—as members of the same family.4