simultaneously, which was not taken into consideration by Kassian-Dybo's research. Ukrainian linguists (Stepan Smal-Stotsky, Ivan Ohienko, George Shevelov, Yevhen Tymchenko, Vsevolod Hantsov, Olena Kurylo) deny the existence of a common Old East Slavic language at any time in the past. According to them, the dialects of East Slavic tribes evolved gradually from the common Proto-Slavic language without any intermediate stages. Linguistic history The following is a summary of the main changes from Proto-Indo-European (PIE) leading up to the Common Slavic (CS) period immediately following the Proto-Slavic language (PS). Satemisation: PIE *ḱ, *ǵ, *ǵʰ → *ś, *ź, *źʰ (→ CS *s, *z, *z) PIE *kʷ, *gʷ, *gʷʰ → *k, *g, *gʰ Ruki rule: Following *r, *u, *k or *i, PIE *s → *š (→ CS *x) Loss of voiced aspirates: PIE *bʰ, *dʰ, *gʰ → *b, *d, *g Merger of *o and *a: PIE *a/*o, *ā/*ō → PS *a, *ā (→ CS *o, *a) Law of open syllables: All closed syllables (syllables ending in a consonant) are eventually eliminated, in the following stages: Nasalization: With *N indicating either *n or *m not immediately followed by a vowel: PIE *aN, *eN, *iN, *oN, *uN → *ą, *ę, *į, *ǫ, *ų (→ CS *ǫ, *ę, *ę, *ǫ, *y). (NOTE: *ą *ę etc. indicates a nasalized vowel.) In a cluster of obstruent (stop or fricative) + another consonant, the obstruent is deleted unless the cluster can occur word-initially. (occurs later, see below) Monophthongization of diphthongs. (occurs much later, see below) Elimination of liquid diphthongs (e.g. *er, *ol when not followed immediately by a vowel). First palatalization: *k, *g, *x → CS *č, *ž, *š (pronounced , , respectively) before a front vocalic sound (*e, *ē, *i, *ī, *j). Iotation: Consonants are palatalized by an immediately following *j: sj, *zj → CS *š, *ž nj, *lj, *rj → CS *ň, *ľ, *ř (pronounced or similar) tj, *dj → CS *ť, *ď (probably palatal stops, e.g. , but developing in different ways depending on the language) bj, *pj, *mj, *wj → *bľ, *pľ, *mľ, *wľ (the lateral consonant *ľ is mostly lost later on in West Slavic) Vowel fronting: After *j or some other palatal sound, back vowels are fronted (*a, *ā, *u, *ū, *ai, *au → *e, *ē, *i, *ī, *ei, *eu). This leads to hard/soft alternations in noun and adjective declensions. Prothesis: Before a word-initial vowel, *j or *w is usually inserted. Monophthongization: *ai,