Jan Baudouin de Courtenay (1845 - 1929)

Jan Niecisław Ignacy Baudouin de Courtenay, also Ivan Alexandrovich Baudouin de Courtenay, was a Polish linguist and Slavist, best known for his theory of the phoneme and phonetic alternations, editor of dictionaries, an active Esperantist and president of the Polish Esperanto Association, and one of the co-founders of the Polish Linguistic Society.