What similarities led Sir William Jones to connect Sanskrit, Greek, and Latin?

In 1786, the British orientalist Sir William Jones proposed that Sanskrit, Greek, and Latin shared a common ancestry.12

Jones was led to this conclusion by observing striking resemblances between these classical languages. Specifically, he noted that they shared a number of items of basic vocabulary and grammatical structures that suggested a genealogical relationship. While Jones provided the initial hypothesis that these languages must have descended from a common source—which likely no longer existed—his observations laid the groundwork for the comparative method in comparative linguistics.132

Later scholars, such as Jacob Grimm, expanded on these findings by identifying systematic phonetic correspondences, which further solidified the understanding of the Indo-European family of languages.13