I don't agree with this mechanic because until the 19 century explorers say that Bulgarians strech to Morava river there were around 200 000 Bulgarians in Serbia around the 1840s. I would like the Ottoman (or any Turkic beylic) to conquer the Balkans and to have the Decision to resettle Bulgarian Population to from Sumadija (west of the Morava) to East Thrace.
I have given evidence about the ethnicity of the Slavs in Kosovo (exept Mitrovica and Pech), around the Morava and Belgrade. And have concluded that they are Bulgarian through
1 Assimilation By Bulgarians in the 9-11 century. And being part of the Bulgarian nationality which forms in the 10th century.
2 Branislav Nushich (a Serb) has given village types and says that The Bulgarian are Centralized and They extend to Kosovo Pole, Morava river.
3 The Kosovo- Morava Dialect is closer to Bulgarian.
About the Smederevo vrach dialect. As i said earlier the Slavs there were part of the Bulgarian ethnos (ethnicity) and Serbia still didn't have so much influence over these lands I think they conquered them in 1296 and it's impossible to assimilate people so fast in the middle ages.
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Also the true Serbian dialects were the one Spoken in Herzegovina and Montenegro. The softer one "ijekavian" .
If the Smederevo-Vrach and Sumadija (I agree that Sumadija one is Serbian) the why did Vuk Karadzic standardize the literary language as the one in Herzegovina instead of the ones in Central Serbia.
Ekavian Dialects became more important later on in the 19 century when Belgrade became the political center. Whereas in the Middle ages the political center was Montenegro, Raška and Herzegovina.
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The light Blue is Smederevo-Vrach and the Dark Blue is Kosovo-Morava (Kosovo-Resava) Dialect
Also there are people speaking the Smederevo-Vrach dialect which were probably medieval Slavic (Bulgarian) population which helped Petar Delian conquer Belgrade. The slavs which were ruled by Ahtum, Kupan and Dyula (Gyula) as well as Salan, Menumorut.
If you disagree please reply