Transitional dialects are spread about the two sides of today's Bulgarian-Serb border and are a gradual transition between Bulgarian and Serbian languages. Bulgarian are those dialects that were spoken inside the borders of Bulgaria before 1918, namely the dialects around Belogradchik, western of Berkovitsa, around Tsaribrod, Trən, Breznik, and Bosilegrad, known as Belogradchik-Trən dialect. Serbian are the dialects west of the border around Knjaževac, Pirot, Leskovac, and Vranja.