text: Western Shtokavian covered the major part of present-day Bosnia and Herzegovina, Slavonia and part of Southern Dalmatia in Croatia. Eastern Shtokavian was dominant in Serbia, easternmost Bosnia and Herzegovina and greater parts of Montenegro. From the 12th century, both dialects started separating further from Chakavian and Kajkavian idioms. According to research of historical linguistics, Old-Shtokavian was well established by the mid-15th century. In this period it was still mixed with Church Slavonic to varying degrees. However, the ultimate development of Western Shtokavian and Eastern Shtokavian was not divergent (like in the case of Chakavian and Kajkavian), but convergent. It was the result of migrations (particularly of Neoshtokavian-Eastern Shtokavian speakers), political-cultural border change and also caused by the Ottoman invasion (since the 16th century). Initially two separate proto-idioms started to resemble each other so greatly that, according to Brozović (1975), "[today] we can no longer speak of an independent Western Shtokavian, but only about the better or weaker preservation of former West Shtokavian features in some dialects of the unique Shtokavian group of dialects".
  As can be seen from the image on the right, originally the Shtokavian dialect covered a significantly smaller area than it covers today, meaning that the Shtokavian speech has spread for the last five centuries, overwhelmingly at the expense of Chakavian and Kajkavian idioms. The modern areal distribution of these three dialects as well as their internal stratification (Shtokavian and Chakavian in particular) is primarily a result of the migrations resulting from the spread of the Ottoman Empire in the Balkans. Migratory waves were particularly strong in the 16th–18th century, bringing about large-scale linguistic and ethnic changes in the Central South Slavic area (see also Great Serb Migrations).