Several centuries ago, comparisons of older languages such as Latin, Greek, Sanskrit, and Gothic showed that similarities among word forms with similar meanings were so systematic as to rule out chance or borrowing as an explanation. Such systematic similarities, it was argued, could only have resulted if the speakers of these languages once formed a community that then broke up as groups of its speakers migrated to different places. Because these languages ranged geographically from India to Europe, they were called Indo-European (abbreviated IE) and their unknown prehistoric ancestor was called Proto-Indo-European (PIE). The Indo-European Languages are divided into families such as Celtic, Germanic (including English), and Slavic, which are traditional groupings of the languages for which IE Texts survive. Study of linguistic and cultural artifacts gives us insight into the early PIE speakers, and comparative-historical methods allow us to track the evolution of these IE languages and cultures.
Indo-European Linguistics discusses linguistic traits of IE languages such as their sounds, word formation, lexicon, and grammar. At the same time that scholars were discovering genetic relations among Indo-European languages such as Welsh, Irish, German, Hindi and Bengali, among Finno-Ugric languages such as Finnish, Hungarian and Estonian, among Kartvelian languages such as Georgian and Mingrelian, or among Altaic languages such as Turkish and Mongolian, some also noticed typological similarities among the linguistic structures of genetically unrelated languages such as Japanese and Turkish. Studies of geographical culture areas such as the ancient Near East further show that culturally-linked regions may share non-genetic language similarities.
Hypotheses about the nature of the prehistoric Proto-Indo-European language are based on comparisons of attested language data. Reconstructed proto-words or roots of words may be related, as shown by Carl D. Buck's semantic categories (see Semantic Fields), and make up part of the IE Lexicon. Semantic categories represented by words in the attested IE languages reflect IE Culture and, more generally, the culture of areas where Proto-IE (further abbreviated PIE) was once spoken.
The LRC's Early Indo-European Online (EIEOL) project is creating lessons to make texts in the older IE languages more accessible, while the Indo-European Typology project begins to index publications about Indo-European by topic, in particular dealing with language typology.
N.B. There are many thousands of publications addressing diverse aspects of Indo-European culture, languages, linguistics, and texts. The books mentioned above, though significant, represent but a tiny fraction of the total. The Journal of Indo-European Studies, among other journals, contains ongoing studies of Indo-European language and culture.