Common Baltic language
It is uneasy to write about Common Baltic, for it is purely reconstructed, no inscriptions or ancient authors writing about it. And sometimes it is had to define, whether this or that feature of modern Baltic languages was developed in Common Baltic, Common Balto-Slavic epoch or even earlier. Comparative linguistics is that teacher which will lead us to the right way.

Balto-Slavic community was dividing into two subfamilies very slowly, there were no states at that time, practically no migrations. Nowadays toponimic terms, names of rivers, lakes and mountains in the territory of modern Belorussia and northern Russia can hardly de determined, if they are of Baltic or Slavic origin. Nevertheless, we know that Balts lived on vast forest and plain lands along the Neman, the Narev, the Daugava rivers. Later they moved towards the Baltic Sea coming into contact with Finnish tribes.

Baltic differed greatly from Common Proto-Indo-European in phonetics. Quite a lot of consonants, particularly labiovelars and palatals, were reconstructed into sibilants like English [sh], [ch], [zh]. These sounds are still very numerous in Lithuanian, and were of the same number in Old Prussian. But  Baltic phonetics can be called conservative as well as the grammar: it preserved diphthongs, it kept division of vowels into long and short, etc. Rich are the vowel and consonant interchanges in the stem, which distinguished both Baltic and Common Slavic languages.

Grammar was much alike Proto-Indo-European. All eight cases of nouns (though there was already a tendency of assimilation of ablative by genitive), three genders, very inflected structure of adjectives. All pronouns were declined as well. The verb held Proto-Indo-European aorist, optative, imperative and conjunctive moods. The infinitive, which was developed already in Balto-Slavic epoch, was formed from the dative of Indo-European verbal noun and had a -ti ending still seen in Lithuanian.

It seems that Common Baltic did not exist long, gradually being divided into western and eastern dialects. This can be explained by the large territory of Baltic inhabitancy. So already at the beginning of AD, Common Baltic had several varieties which soon became separate Baltic languages.