It would not have been possible to establish the existence of the Indo-European language family if scholars had not compared the systematically recurring resemblances among European languages and Sanskrit, the oldest language of the Indian subcontinent that left many written documents. The common origin of European languages and Sanskrit was first proposed by Sir William Jones(1746-1794). Systematic comparisons between these languages by Franz Bopp supported this theory and laid the foundation for postulating that all Indo-European languages descended from a common ancestor, Proto-Indo-European (PIE), thought to have been spoken before 3,000 B.C. It then split into different branches which, in turn, split into different languages in the subsequent millennia.
Since PIE left no written records, historical linguists construct family trees, an idea pioneered by August Schleicher, on the basis of the comparative method. The comparative method takes shared features among languages and uses procedures to establish their common ancestry. It is not the only method available but is one that has been most widely used. The examples below show how this method actually works with some Indo-European languages.