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