Balto-Slavic languages
The Balto-Slavic languages is a branch of the Indo-European family of languages. It traditionally comprises 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 research in the field of comparative Balto-Slavic accentology. Even though some linguists still reject a genetic relationship, most scholars accept that Baltic and Slavic languages experienced a period of common development. This view is also reflected in most modern standard textbooks on Indo-European linguistics. Gray and Atkinson's (2003) application of language-tree divergence analysis supports a genetic relationship between the  Baltic and Slavic languages, dating the split of the family to about 1400 BCE. Internal classification. The traditional division into two distinct sub-branches (i.e. Slavic and Baltic) is mostly upheld by scholars who accept Balto-Slavic as a genetic branch of Indo-European. There is a general consensus that the Baltic languages can be divided into East Baltic (Lithuanian, Latvian) and West Baltic (Old Prussian). The internal diversity of Baltic points at a much greater time-depth for the breakup of the Baltic languages in comparison to the Slavic languages. "Traditional" Balto-Slavic tree model This bipartite division into  Baltic and Slavic was first challenged in the 1960s, when Vladimir Toporov and Vyacheslav Ivanov observed that the apparent difference between the "structural models" of the Baltic languages and the Slavic languages is the result of the innovative nature of Proto-Slavic, and that the latter had evolved from an earlier stage which conformed to the more archaic "structural model" of the Proto-Baltic dialect continuum. Frederik Kortlandt (1977, 2018) has proposed that West Baltic and East Baltic are in fact not more closely related to each other than either of them is related to Slavic, and Balto-Slavic therefore  can be split into three equidistant branches: East Baltic, West Baltic and Slavic. Alternative Balto-Slavic tree model Although supported by a number of scholars, Kortlandt's hypothesis is still a minority view. Some scholars accept Kortlandt's division into three branches as the default assumption, but nevertheless believe that there is sufficient evidence to unite East Baltic and West Baltic in an intermediate Baltic node. The tripartite split is supported by glottochronologic studies by V. V. Kromer, whereas two computer-generated family trees (from the early 2000s) that include Old Prussian have a Baltic node parallel to the Slavic  node. Historical expansion. The sudden expansion of Proto-Slavic in the sixth and the seventh century (around 600 CE, uniform Proto-Slavic with no detectable dialectal differentiation was spoken from Thessaloniki in Greece to Novgorod in Russia) is, according to some, connected to the hypothesis that Proto-Slavic was in fact a "koiné" of the Avar state, i.e. the language of the administration and military rule of the Avar Khaganate in Eastern Europe. In 626, the Slavs, Persians and Avars jointly attacked the Byzantine Empire and participated in the Siege of Constantinople. In that campaign, the Slavs fought under Avar  officers. There is an ongoing controversy over whether the Slavs might then have been a military caste under the khaganate rather than an ethnicity. Their language—at first possibly only one local speech—once koinéized, became a "lingua franca" of the Avar state. This might explain how Proto-Slavic spread to the Balkans and the areas of the Danube basin, and would also explain why the Avars were assimilated so fast, leaving practically no linguistic traces, and that Proto-Slavic was so unusually uniform. However, such a theory fails to explain how Slavic spread to Eastern Europe, an area that  had no historical links with the Avar Khanate. That said, the Avar state was later replaced by the definitively Slavic state of Great Moravia, which could have played the same role. It is also likely that the expansion of Slavic occurred with the assimilation of Iranic-speaking groups such as the Sarmatians, who quickly adopted Proto-Slavic due to speaking related Indo-European satem languages, in much the same way Latin expanded by assimilating the Celtic speakers in continental Western Europe and the Dacians. That sudden expansion of Proto-Slavic erased most of the idioms of the Balto-Slavic dialect  continuum, which left us today with only two groups, Baltic and Slavic (or East Baltic, West Baltic, and Slavic in the minority view). This secession of the Balto-Slavic dialect ancestral to Proto-Slavic is estimated on archaeological and glottochronological criteria to have occurred sometime in the period 1500–1000 BCE. Hydronymic evidence suggests that Baltic languages were once spoken in much wider territory than the one they cover today, all the way to Moscow, and were later replaced by Slavic. Shared features of the Balto-Slavic languages. The degree of relationship of the Baltic and Slavic languages is indicated by  a series of common innovations not shared with other Indo-European languages, and by the relative chronology of these innovations which can be established. The Baltic and Slavic languages also share some inherited words. These are either not found at all in other Indo-European languages (except when borrowed) or are inherited from Proto-Indo-European but have undergone identical changes in meaning when compared to other Indo-European languages. This indicates that the Baltic and Slavic languages share a period of common development, the Proto-Balto-Slavic language. Common sound changes. Common Balto-Slavic innovations include several other changes , which are also shared by several other Indo-European branches. These are therefore not direct evidence for the existence of a common Balto-Slavic family, but they do corroborate it. Shared vocabulary. Some examples of words shared among most or all Balto-Slavic languages: Despite lexical developments exclusive to Balto-Slavic and otherwise showing evidence for a stage of common development, there are considerable differences between the vocabularies of Baltic and Slavic. Rozwadowski noted that every semantic field contains core vocabulary that is etymologically different between the two branches. Andersen prefers a dialect continuum model where the northernmost dialects developed  into Baltic, in turn, the southernmost dialects developed into Slavic (with Slavic later absorbing any intermediate idioms during its expansion). Andersen thinks that different neighboring and substratum languages might have contributed to the differences in basic vocabulary. Criticism. Lithuanian linguist and scholar Antanas Klimas has criticized Oswald Szemerényi's arguments, which are in favour of the Balto-Slavic theory. His counterarguments regarding the plausible phonetical, phonological, and morphological similarities between the Baltic and Slavic languages had scrutinized the arguments of O. Szemerényi and concluded the following: He had also noted that: Morphological differences. The opponents of the Balto-Slavic theory  had presented morphological properties that, according to them, prove that the Proto-Balto-Slavic language did not exist: