Solun (Thessaloniki) no relation Bulgarian
Bulgarian
In Bulgarian the different reflexes of the yat form the so-called yat border (ятова граница), running approximately from Nikopol on the Danube to Solun (Thessaloniki) on the Aegean Sea. West of that isogloss, old yat is always realized as /ɛ/ (analogous to the Ekavian Serbian dialects further west). East of it, the reflexes of yat prototypically alternate between /ja/ or /ʲa/ (in stressed syllables when not followed by a front vowel) and /ɛ/ (in all other cases). The division of the dialects of the Eastern South Slavic into western and eastern subgroup running along the yat border is the most important dividing isogloss there.Some older Serbian scholars believed that the yat border divides the Serbian and Bulgarian languages. However, modern Serbian linguists such as Pavle Ivic have accepted that the main isoglosses bundle dividing Eastern and Western South Slavic runs from the mouth of the Timok river alongside Osogovo mountain and Sar Mountain. On the other hand, in Bulgaria the Timok-Osogovo-Sar isogloss is considered the eastern most border of the broader set of the transitional Torlakian dialects, described often as part of the Eastern South Slavic, (i.e. Bulgarian/Macedonian). 
In 1870 Marin Drinov, who played a decisive role in the standardization of the Bulgarian language, rejected the proposal of Parteniy Zografski and Kuzman Shapkarev for a mixed eastern and western Bulgarian/Macedonian foundation of the standard Bulgarian language. From the late 19th century until 1945, the standard Bulgarian orthography did not reflect this alternation and used the Cyrillic letter yat for both /ja/ and /e/ in alternating roots. This was regarded as a way to maintain unity between Eastern and Western Bulgarians, as much of what was then, and is now, seen as Western Bulgarian dialects were in the Late Middle Ages partially under Serbian control. In 1945 the Yat-letter was removed from the Bulgarian alphabet and the spelling was changed to conform to the Eastern pronunciation. Some examples of the alternation in the standard language follow:
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