Interrogative pronouns kaj (what) and kdo (who) are rather peculiar within the range of Slavic languages. In principle, the Slovenian pronoun kaj corresponds to the Russian pronoun čto and the Slovenian pronoun kdo to the Russian pronoun kto. The form kaj of the first pronoun is also known to the Croatian Kajkavian and Buzet dialects as opposed to the form ča in the Croatian Chakavian dialect and čo in the Slovak language, while the form kdo of the second pronoun also belongs to both the Croatian Kajkavian dialect and the Czech language. The above-mentioned peculiarities are connected with a complicated development of Proto-Slavic pronouns čь and kъ. The form kaj otherways reminds of the particle -kaj optionally hanged to the place adverb semkaj after imperative vleci what is comparable to the Russian imperative construction duj-ka… sjuda.