The article analyses the origin of the common Mari o (occurring in the Hill, North-Western, Meadow and Eastern Mari dialects), which corresponds to the Mordvin and Finnic а in common Finno-Ugric and genuine Mari words, as well as the opposition of the Hill and North-Western Mari а ~ Meadow and Eastern Mari o which manifests itself mostly in borrowings from the Bulgar-Chuvash language and partly in some borrowings from the Tatar language. It is concluded that the process of transformation of the Finnic-Volgaic *a into the common Mari o (~ Mordvin а) must be distinguished from the development of the а ~ o opposition in certain Turkic loanwords used in Mari dialects, which is a later process. Nevertheless, the opposition of а ~ o in Mari dialects, despite being mainly manifested in Turkic loanwords, is not directly linked either with the possible opposition of а ~ o/ô/u in Bulgar dialects or with the opposition а ~ a° between Mishar and Kazan Tatar dialects. This is proved by the manifestation of the opposition а ~ o in those Chuvash loanwords occurring in Mari dialects, which could not originally have had it.
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