Languages are always changing, and all modern languages derive from ancient languages. Historical linguistics, the study of languages over time, is one of the great intellectual achievements of the nineteenth century. In the late eighteenth century Western scholars became familiar with Sanskrit and realized that it bore a striking resemblance to Greek and Latin. They theorized that these three languages were all derived from a language which is no longer spoken and which has left no written records. They called this language Proto-Indo-European (PIE), and the many languages derived from Proto-Indo-European constitute the Indo-European family. This family includes the Germanic, Slavic, Romance, Celtic, and Indo-Iranian languages. By comparing the differing forms of words in these daughter languages it was possible to reconstruct a large part of PIE and to show the regularity of the sound changes which have produced the many languages derived from PIE. Historical linguisticsis the most scientific part of philology, but it can also be a great entertainment, and many people enjoy learning something about the history of the words we use.