In this article the origin of the perfective participle with the '-(w)szy' ending in the Polish language of the northeast borderlands is discussed. The author, quoting similar cases concerning this participle from the Polish dialects of the Suwalki, Punsk and Sejny regions, suggests the historical influence of the Lithuanian language on his origin. In addition, the fieldwork research carried by the author in the Trakai region in Lithuania and in the Ivjanec region in Belarus got interesting results: in the Polish language of the Ivjanec region in Belarus this participle did not appear once.
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