Monophthongization of diphthongs in Proto-Slavic
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Monophthongization of diphthongs is a Proto-Slavic sound change in which diphthongs turn into vowels. It is one of the key events in the chronology of the Proto-Slavic language. The monophthongization of diphthongs restructured the Proto-Slavic language with a strong influence on its morphophonology.
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 the first palatalization or the second.
- *ai > *ě₂, *i₂ - Early Slavic *snaigu > *sněgъ > Serbo-Croatian snijeg
- *au > *u - Early Slavic *saušu > *suxъ > Serbo-Croatian suh
- *ei > *i₁ - Early Slavic *kreivu > *krivъ > Serbo-Croatian kriv
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.
- The o-stem nominative plural (Old Church Slavonic člověkъ, člověci), reflecting the original Proto-Indo-European pronominal ending *-oy, also seen in Lithuanian -ai.
- The thematic imperative infix (Old Church Slavonic rešti, rekǫ, reci), reflecting the original Proto-Indo-European infix *-oyh₁-.