The Lithuanian definite adjective construction originates from the suffixation of the Proto-
Indo-European *ye/o- relative pronoun to simple form adjectives (baltas vs. baltasis). Although
this category has formal parallels in virtually every member of Balto-Slavic, the facts
of its historical development are largely lost due to the relatively late attestation of both
Baltic and Slavic. Evidence from Old Lithuanian, however, suggests that grammaticalization
of the definite marker as adjectival morphology occurred exceptionally late, likely after the
split of Baltic and Slavic. Furthermore, Old Lithuanian shows signs of the definite marker
also having encliticized to nominal modifiers, most often found in the locative case (t˙evas
dangujejis, father in heaven).

The present thesis explores the implications of how the relative pronoun, and by extension
relative clauses in Proto-Indo-European, could undergo development to form the adjectival
construction of Modern Lithuanian. First, the analysis motivates the origins of the definite
marker from the domain of syntax by relying on establishing a structural parallel in the historical
development of the reflexive particle si— known to have been a Wackernagel’s Law
clitic by comparison with facts of Slavic—with that of the deverbal adjectives of Old Lithuanian
when taken in their definite form (pajoprasto vs prastojo, nusiprausti vs praustis). Both
these particles’ complementary distribution as prefix and suffix suggests earlier sensitivity to
elements of clausal syntax. On this evidence, a reconstruction for Proto-Balto-Slavic clauses
is posited and the internal evidence is further corroborated by the use of the comparative
method on Balto-Slavic and Indo-Iranian, the latter still attesting the Proto-Indo-European
relative pronoun in its original use.

Finally, the early development of the predecessor to the definite marker jis is explored for
the period from Proto-Indo-European up until latest Proto-Balto-Slavic. Using the history
of the Iranian ezafe as a case study for a parallel development in early Balto-Slavic, a careful
account is given of the reanalysis of adjectival and nominal predicates in relative clauses to
justify the formation of both definite adjectives and dangujejis type construction.