The article is devoted to describing little-known facts about the studies of criminal dialects (argot) at the beginning of the 20th century. The author focuses on the unreleased manuscript by P. P. Ilyn “Studying criminals’ jargon” (1912). Prof. Grachev proves that Jan Baudouin de Courtenay did fundamental analytical research of argot. The remarkable fact is that Jan de Courtenay used information acquired from the prisoners (P. P. Ilyn and V. F. Trachtenberg), who had necessary linguistic competencies, for his study. Geographically, the area of de Courtenay’s research is quite large: from St. Petersburg to Irkutsk. Furthermore, it is important to mention another de Courtenay’s contribution to studying argot: he was an editor of the dictionary “Blatnaya Muzika, Jargon”. De Courtenay’s comments about the “hangman humour”, which is included in the argot, are interesting and ingenious. The author of the article emphasizes that Jan Baudouin de Courtenay was the first person to study Russian argot.