Ukrainian is an East Slavic language. It is the official state language of Ukraine and the principal language of Ukrainians. Written Ukrainian uses a variant of the Cyrillic script. The Ukrainian language traces its origins to the Old East Slavic of the early medieval state of Kievan Rus'. From 1804 until the Russian Revolution, the Ukrainian language was banned from schools in the Russian Empire, of which the biggest part of Ukraine was a part at the time. It has always maintained a sufficient base in Western Ukraine, where the language was never banned, in its folklore songs, itinerant musicians, and prominent authors. The standard Ukrainian language is regulated by the National Academy of Sciences of Ukraine, particularly by its Institute for the Ukrainian Language, Ukrainian language-information fund, and Potebnya Institute of Language Studies. Lexically, the closest to Ukrainian is Belarusian, followed by Polish, Serbo-Croatian, Slovak and Russian. The Ukrainian language retains a degree of mutual intelligibility with Belarusian and Russian.