Phonotactics
Most syllables in Middle Common Slavic were open. The only closed syllables were those that ended in a liquid (*l or *r), forming liquid diphthongs, and in such syllables, the preceding vowel had to be short. Consonant clusters were permitted, but only at the beginning of a syllable. Such a cluster and was syllabified with the cluster entirely in the following syllable, contrary to the syllabification rules that are known to apply to most languages. For example, *bogatĭstvo "wealth" was divided into syllables as *bo-ga-tĭ-stvo, with the whole cluster -stv- at the beginning of the syllable.
By the beginning of the Late Common Slavic period, all or nearly all syllables had become open as a result of developments in the liquid diphthongs. Syllables with liquid diphthongs beginning with an o or e had been converted into open syllables, e.g. *tort became *trot, *trat or *torot. The main exception are the Northern Lekhitic languages (Kashubian, extinct Slovincian and Polabian) only with lengthening of the syllable and no metathesis (*tart, e.g. PSl. *gord > Csb. gard; > Plb. *gard > gord). In West Slavic and South Slavic, liquid diphthongs beginning with ĭ or ŭ had likewise been converted into open syllables by converting the following liquid into a syllabic sonorant (palatal or non-palatal according to whether an ĭ or ŭ preceded). This left no closed syllables at all in these languages. The South Slavic languages, as well as Czech and Slovak, tended to preserve the syllabic sonorants, but in the Lekhitic languages (e.g. Polish), they fell apart again into vowel-consonant or consonant-vowel combinations. In East Slavic, the liquid diphthongs in ĭ or ŭ may have likewise become syllabic sonorants, but if so, the change was soon reversed, suggesting that it may never have happened in the first place.
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