EN
In Slavic linguistics, it was commonly accepted from the beginning of the 19th century that the oral articulation of Old Church Slavonic nasal vowels didn't differ from the articulation of the oral vowels o and e. This opinion was probably based on the fact that the only language that had retained nasal vowels - the Polish language - had this kind of articulation. It was believed that the Old Church Slavonic language didn't differ much from the Proto-Slavic language, and this articulation was accepted for the reconstruction of the phonology of that language. The vowel jat' (e) was treated as an open e, which in some languages and dialects has developed in certain environments into a (Polish and Bulgarian), in other cases into e (this change operated also as an environmentally unconditioned one in Serbian, Macedonian, Russian and Belarusian), but also into i (in the Ukrainian language and Croatian dialects), into the diphthong ie / je (in Croatian), or has remained a distinct close phoneme ('e' in Sorbian languages). The back nasal vowel has in most of the languages changed into u, but in some of them into o (Slovenian), into a (Bulgarian) and further into a (Macedonian). The front nasal vowel changed into e (in the remaining South Slavic languages) or into a (East Slavic languages, Upper Sorbian and initially Czech and Slovak), whereas in Lower Sorbian it merged with the jat' reflexes. In literary Polish, (after long and short nasal vowels had merged) since the 16th century, we have reflexes articulated as e () and o (). The views concerning the articulation of nasal vowels and jat' begun to change under the influence of G.Y. Shevelov's works (1964), which - based on previous borrowings of Slavic words into non-Slavic languages and vice-versa - marked a significant turning point in views on the phonology of the Late Proto-Slavic period. The dialectal materials (taken mainly form works published in connection to the Slavic Linguistic Atlas - OLA) presented in this paper enable a preliminary revision of the views concerning the realisation of Early Proto-Slavic nasal vowels and diphthongs, from which jat' derived. In nearly the entire South Slavic area, a figures among the reflexes of the front nasal vowel and of jat', which points to the common development of Proto-Slavic front diphthongs (containing i and nasal vowels), and therefore to an open articulation of the initial jat'. Therefore the so called jat' umlaut (in Bulgarian or Polish) is not an umlaut, but the original pronunciation before non-palatal consonants. The parallel development of back diphthongs (containing u as well as and nasal vowels) is even more visible: in case of diphthongs containing u, on the entire area, and where the nasal diphthongs are concerned, covering the great majority of Slavic dialects and languages.