Lithuanian is one of two living Baltic languages, along with Latvian. An earlier Baltic language, Old Prussian, was extinct by the 18th century; the other Western Baltic languages, Curonian and Sudovian, became extinct earlier. Some theories, such as that of Jānis Endzelīns, considered that the Baltic languages form their own distinct branch of the family of Indo-European languages, but the most widely accepted opinion is the one that suggests the union of Baltic and Slavic languages into a distinct sub-family of Balto-Slavic languages amongst the Indo-European family of languages. Such an opinion was first represented by the likes of August Schleicher, and to a certain extent, Antoine Meillet. Endzelīns thought that the similarity between Baltic and Slavic was explicable through language contact while Schleicher, Meillet and others argued for a genetic kinship between the two families.
An attempt to reconcile the opposing stances was made by Jan Michał Rozwadowski. He proposed that the two language groups were indeed a unity after the division of Indo-European, but also suggested that after the two had divided into separate entities (Baltic and Slavic), they had posterior contact. The genetic kinship view is augmented by the fact that Proto-Balto-Slavic is easily reconstructible with important proofs in historic prosody. The alleged (or certain, as certain as historic linguistics can be) similarities due to contact are seen in such phenomena as the existence of definite adjectives formed by the addition of an inflected pronoun (descended from the same Proto-Indo-European pronoun), which exist in both Baltic and Slavic yet nowhere else in the Indo-European family (languages such as Albanian and the Germanic languages developed definite adjectives independently), and that are not reconstructible for Proto-Balto-Slavic, meaning that they most probably developed through language contact.
Vyacheslav Vsevolodovich Ivanov and Vladimir Toporov believed in the unity of Balto-Slavic, but not in the unity of Baltic. In the 1960s, they proposed a new division, that into East-Baltic (Lithuanian and Latvian), West-Baltic (Old Prussian), and Slavic. The Ivanov-Toporov theory is gaining ground among students of comparative-historic grammar of Indo-European language, and seems to be replacing the previous two stances in most P-I-E textbooks.