The first stage of Indo-European study was the broad classification work that established many of the well-accepted groups of Indo-European languages. This was done by the 1830s. Since then, a few other languages of the family have been added. The work of subgrouping is more difficult and there are still points of disagreement among scholars.
In the 1850s, scholars began to reconstruct sounds and words of the presumed ancestral language from which all Indo-European languages are descended. This reconstruction of Proto-Indo-European is necesssarily partial. The actual language was a normal language with tens of thousands of vocabulary items and a full grammar, but all that can be reconstructed of it is a few thousand words and some basic grammatical properties.
The original homeland of the speakers of Proto-Indo-European (PIE) is not known for certain, but many scholars believe it lies somewhere around the Black Sea. Most of the subgroups diverged and spread out over much of Europe and the Near East and northern Indian subcontinent during the fourth and third millennia BC.
Since that time, enormous amounts of work have been done on the structure and vocabulary. The ancient Indo-European languages preserved in writing, namely Greek, Latin, and Sanskrit, are inflected languages. The inflections can be used to reconstruct ancient inflections, and it is commonly accepted that Proto-Indo-European had a number of classes of nouns and a case system slightly more elaborate than that of Latin and Greek. The case and gender systems are said to be best preserved, among the modern I.E. languages, in Lithuanian.
Discussion of PIE culture in the 20th century was stalled by its association with the racist doctrines of National Socialism. But recent scholarship has returned to the question based on archaeological finds (Gimbutas) and the history of agriculture (Renfrew).
As PIE is not directly attested, all PIE sounds and words are reconstructed (using the comparative method). The standard convention is to mark reconstructed (and therefore more or less hypothetical) forms with an asterisk, e.g. *wodr 'water', *k^wo:n 'dog', *trejes 'three (masculine)', etc. Many of the words in the modern Indo-European languages are derived from such "protowords" via regular sound change.
One example of such regular sound change is Grimm's Law, discovered about 1820 by Jakob Grimm, of fairy-tale fame. It establishes a set of regular correspondences between early Germanic stops and fricatives and the stop consonants of certain other Indo-European languages. As formulated nowadays, Grimm's Law describes the development of inherited Proto-Indo-European (PIE) stops in Proto-Germanic (PGmc, the common ancestor of the Germanic branch of the Indo-European family). It consists of three parts: