Because contact between the Balts and Slavs from the time of Proto-Indo-European was never broken off, it is understandable that Baltic and Slavic should share more linguistic features than any of the other Indo-European languages. Thus, Indo-European *eu passed to Baltic jau and Common Slavic *jau (which became ju)—e.g., Lithuanian liáudis “people,” Latvian ļáudis, Old Church Slavonic ljudije. Tonal correspondences are found between Lithuanian and Serbo-Croatian (a Slavic language of Yugoslavia), and there are also similarities in stress; e.g., Lithuanian dūmai “smoke” and Russian dym have the stress on the root, as do Lithuanian rañką “hand” (accusative singular) and Russian rúku, while both Lithuanian rankà “hand” (nominative singular) and Russian ruká are stressed on the second syllable.

Baltic and Slavic have specific morphological features in common. Among them, for example, is the genitive plural form. In Lithuanian, mū´sų “of us” (= Latvian mūsu), evolved from the older form *nūsōn, which comes from Baltic *nōsōn and corresponds to the genitive plural form in Common Slavic, *nōsōn, from which developed Old Church Slavonic nasŭ “of us.” Baltic also shares some syntactic features with Slavic; e.g., the genitive case is used in place of the accusative with verbs expressing negation (Lithuanian jis nieko nežino “he does not know anything,” Latvian viṇš nekā nezin, Russian on ničego ne znajet). There are also many lexical items common to Baltic and Slavic. More than 100 words are common in their form and meaning to Baltic and Slavic alone, among them Lithuanian bėgu “I run,” Latvian bēgu, Old Church Slavonic běgǫ; Lithuanian líepa “linden tree,” Latvian liẽpa, Old Prussian lipe, Old Church Slavonic lipa; Lithuanian rãgas “horn,” Latvian rags, Old Prussian ragis, Old Church Slavonic rogŭ.

In addition to these features common to all the Baltic and Slavic languages, there are certain quite archaic features that Slavic shares with Lithuanian and Latvian but not with Old Prussian. The most striking example is the genitive singular ending in Lithuanian viĺk-o = Latvian vìlk-a “of a wolf,” which comes from Baltic *-ō, historically paralleled by the genitive singular ending in Common Slavic *vǐlk-ā. Old Prussian, however, has a different ending for the same inflection (deiw-as “of God”). In some instances the Slavic languages, differing from Lithuanian and Latvian, come closer to Old Prussian; e.g., the Prussian possessive pronouns mais “my, mine,” twais “your, yours,” swais “one’s own” are different from Lithuanian mãnas, tãvas, sâvas and from Latvian mans, tavs, savs but similar to Old Church Slavonic mojǐ, tvojǐ, svojǐ.

It is possible to conclude that there was close contact between the Baltic and Slavic protolanguages at the time when they began to develop as independent groups (i.e., from about the 2nd millennium bce) and that the Proto-Slavic area might have been a part of peripheral Proto-Baltic, although a specific part. That is, Proto-Slavic at that time was in direct contact with both the corresponding dialects of the peripheral Proto-Baltic area (e.g., with Proto-Prussian) and the corresponding dialects of the central Proto-Baltic area. All this shows that the Proto-Slavic area of that time (south of the Pripyat River) was much smaller than the Proto-Baltic area. Proto-Slavic began to develop as a separate linguistic entity in the 2nd millennium bce and was to remain quite unified for a long time to come. Proto-Baltic, however, besides developing into an independent linguistic unit in the 2nd millennium bce, also began gradually to split. Among other things, the size of the Proto-Baltic area had an influence on the development of Proto-Baltic in that it considerably reduced contact between its dialects (see also Slavic languages).

Development of the individual Baltic languages

By the middle of the 1st millennium bce, the Proto-Baltic area was already sharply split into dialects. From the middle of the 1st millennium ce, the Baltic language area began to shrink considerably; at that time the greater part of Baltic territory, the eastern part, began to be inhabited by Slavs migrating from the south. The Balts there were gradually assimilated by the Slavs; complete assimilation probably occurred around the 14th century. One of these Baltic tribes, the Galindians (Goljadĭ), is mentioned in a chronicle as late as the 12th century. The protolanguage of the so-called Eastern Balts split into Lithuanian and Latvian (Latgalian) around the 7th century. The other languages of the so-called Eastern Balts became separated probably at the same time. Selonian and Semigallian could have been transitional languages between Lithuanian and Latvian. Only Curonian, which some consider to be a transitional language between East and West Baltic, might have developed somewhat earlier. Moreover, the name of the Curonians occurs in historical sources earlier (853 ce: Latin Cori) than the names of the other tribes of the so-called Eastern Balts.

Old Prussian

In historical sources the Prussians are called Aistians from the 1st century ce (by Tacitus) until the 9th century ce (by the Anglo-Saxon seafarer Wulfstan). They are first referred to by their own name (by a Bavarian geographer using the form Bruzi, “Prussians”) in the 9th century ce. About 1230 the Teutonic Order began to plunder the lands of the Prussians and finally conquered them and the Yotvingians (Suduvians) in 1283. From that time the slow extinction of the two Baltic groups began, with the Germanization of the Prussians being completed at the beginning of the 18th century.

The earliest Old Prussian (and, for that matter, Baltic) written record is a German-Prussian vocabulary—the so-called Elbing vocabulary, compiled about 1300 and extant in a copy dated around 1400. This vocabulary, consisting of 802 Old Prussian words (and the same number of German words), was written in a South Prussian dialect (in Pomesania). Somewhat poorer than the Elbing vocabulary is the vocabulary compiled by Simon Grunau, consisting of 100 Old Prussian (and German) words, written between 1517 and 1526. The most important Old Prussian written records are the three catechisms of the 16th century based on the dialects of Sambia and translated from the German; the first two catechisms, which are very short and anonymous, date from 1545, and the third catechism, or Enchiridion, dates from 1561 and was translated by Abelis Vilis (Abel Will), a pastor of the church at Pobeten (Pabec̆iai; modern Romanovo). The language of all the Old Prussian catechisms is rather poor: the translations are excessively literal, and there are many errors in language and orthography. In spite of this, it is from these Old Prussian catechisms that scholars can learn most about the Old Prussian language.