Mikołaj Habdank Kruszewski, was a Polish linguist active in the Russian Empire, most significant as the co-inventor of the concept of the phoneme. From 1883, he was a professor at Kazan University. His notable works include On Sound Alternation (1881) and Outline of Linguistic Science (1883). The former is the introduction to his master's thesis on morphophonemic alternation in Old Slavic and the latter is his doctoral thesis.