Which shared features connect Baltic and Slavic languages?

Baltic and Slavic languages share more linguistic features than any other branches of the Indo-European languages, a connection attributed by many scholars to a common ancestral language or prolonged historical contact.12

Shared features include:

• Phonology: The Proto-Indo-European sound eu evolved into jau in both Baltic and Common Slavic. There are also notable correspondences in stress patterns and tone, such as the root-stressed dūmai (Lithuanian) and dym* (Russian).1
• Morphology: The two groups share specific grammatical forms, such as the genitive plural ending (e.g., Lithuanian mūsu and Old Church Slavonic nasŭ). Additionally, Lithuanian and Latvian share a genitive singular ending with Common Slavic that is not found in Old Prussian.1
• Syntax: Both groups utilize the genitive case instead of the accusative when using verbs that express negation.1
• Vocabulary: There are more than 100 lexical items common to both branches in form and meaning, including terms for nature and anatomy, such as the words for "linden tree" (lipa) and "horn" (rogŭ).1

These similarities suggest that the Slavic protolanguage may have developed from a peripheral area of the Proto-Baltic domain.1