How are Baltic languages related to Slavic languages?

The relationship between Baltic and Slavic languages is a subject of significant linguistic study, often categorized under the Balto-Slavic hypothesis. Scholars who support this hypothesis argue that these groups share a common ancestor that diverged from Proto-Indo-European, while others suggest their similarities stem from long-term contact and parallel development.1

Because contact between these groups was never fully broken, they share more linguistic features than any other Indo-European languages. Key areas of overlap include:2

• Morphology and Syntax: They share specific grammatical structures, such as the genitive plural form and the use of the genitive case with negated verbs.2
• Vocabulary: More than 100 words are unique to Baltic and Slavic in both form and meaning, such as terms for "friend," "wax," and "horn".32
• Phonology: Both groups exhibit shared sound shifts, such as the development of the dative plural ending -m- and specific tonal and stress correspondences.32

Evidence suggests that Proto-Slavic may have originated as a peripheral part of the broader Proto-Baltic area before emerging as a distinct entity in the 2nd millennium BCE.2