The Balto-Slavic languages form a branch of the Indo-European family of languages, traditionally comprising the Baltic and Slavic languages. Baltic and Slavic languages share several linguistic traits not found in any other Indo-European branch, which points to a period of common development. Although the notion of a Balto-Slavic unity has been contested (partly due to political controversies), there is now a general consensus among specialists in Indo-European linguistics to classify Baltic and Slavic languages into a single branch, with only some details of the nature of their relationship remaining in dispute. A Proto-Balto-Slavic language is reconstructable by the comparative method, descending from Proto-Indo-European by means of well-defined sound laws, and from which modern Slavic and Baltic languages descended. One particularly innovative dialect separated from the Balto-Slavic dialect continuum and became ancestral to the Proto-Slavic language, from which all Slavic languages descended. Historical dispute The nature of the relationship of the Balto-Slavic languages has been the subject of much discussion from the very beginning of historical Indo-European linguistics as a scientific discipline. A few are more intent on explaining the similarities between the two groups not in terms of a linguistically "genetic" relationship, but by language contact and dialectal closeness in the Proto-Indo-European period. Baltic and Slavic share many close phonological, lexical, morphosyntactic and accentological similarities (listed below). The early Indo-Europeanists Rasmus Rask and August Schleicher (1861) proposed a simple solution: From Proto-Indo-European descended Balto-German-Slavonic language, out of which Proto-Balto-Slavic (later split into Proto-Baltic and Proto-Slavic) and Germanic emerged. Schleicher's proposal was taken up and refined by Karl Brugmann, who listed eight innovations as evidence for a Balto-Slavic branch in the Grundriss der vergleichenden Grammatik der indogermanischen Sprachen ("Outline of the Comparative Grammar of the Indo-Germanic Languages"). The Latvian linguist Jānis Endzelīns thought, however, that any similarities among Baltic and Slavic languages resulted from intensive language contact, i.e. that they were not genetically more closely related and that there was no common Proto-Balto-Slavic language. Antoine Meillet (1905, 1908, 1922, 1925, 1934), a French linguist, in reaction to Brugmann's hypothesis, propounded a view according to which all similarities of Baltic and Slavic occurred accidentally, by independent parallel development, and that there was no Proto-Balto-Slavic language. In turn, the Polish linguist Rozwadowski suggests that the similarities among Baltic and Slavic languages are a result of both a genetic relationship and later language contact. Thomas Olander corroborates the claim of genetic relationship in his