What is "jcuken"

JCUKEN (ЙЦУКЕН, also known as YCUKEN, YTsUKEN and JTSUKEN) is the main Cyrillic keyboard layout for the Russian language in computers and typewriters. Earlier in Russia JIUKEN (ЙІУКЕН) layout was the main layout, but it was replaced by JCUKEN when the Russian alphabet reform of 1917 removed the letters Ѣ and І.

The Mongolian keyboard uses a modified version of JCUKEN, called "FCUZHEN" , where letters specific to Russian are replaced by letters that see more use in Mongolian.