Monophthongization of diphthongs is aProto-Slavicsound change in whichdiphthongs turn intovowels. It is one of the key events in the chronology of the Proto-Slavic language. Themonophthongization of diphthongs restructured the Proto-Slavic language with a strong influence on itsmorphophonology.
Proto-Balto-Slavic, the ancestor of Proto-Slavic, possessed three vocalic diphthongs:*ai,*au and*ei. In Proto-Slavic, these were monophthongized as follows, with the subscript indicating whether the vowels trigger thefirst palatalization or thesecond.
The fourth Proto-Indo-European vocalic diphthong,*eu, had already become*jau in Proto-Balto-Slavic. It then developed into*ju in Proto-Slavic, following the same development as for*au. The unrounding of older long*ū to Slavic*y had already taken place by the time of the monophthongization; the new*u filled the gap left by it. The first palatalization had also taken place, as the new vowels denoted as*ě₂ and*i₂ did not trigger it.
While most cases of older*ai developed into*ě₂, some inflectional endings appear to have developed*i₂ instead. It is unclear what factors are involved in triggering one reflex versus the other.
George Y. Shevelov dates the monophthongization of diphthongs to the 5th – 7th century AD.[1]Zdzisław Stieber dates the monophthongization of diphthongs to the 5th or 6th century AD.[2]
By the time theSlavic migration to theBalkans began, the monophthongization of diphthongs had not yet taken place, as evidenced by the borrowing of theLatin namePoetovio as the SlavicPtuj.
Subsequent toponyms show the borrowing of the Latin [au] as [ov], which indicates the completion of the process of monophthongization of diphthongs by the time these names were borrowed, for example:[1][2]