Ukrainian (, ) is an East Slavic language of the Indo-European language family, spoken primarily in Ukraine. It is the native language of the Ukrainians. Written Ukrainian uses the Ukrainian alphabet, a variant of the Cyrillic script. The standard Ukrainian language is regulated by the National Academy of Sciences of Ukraine (NANU; particularly by its Institute for the Ukrainian Language), the Ukrainian language-information fund, and Potebnia Institute of Linguistics. Comparisons are often drawn to Russian, another East Slavic language, but there is more mutual intelligibility with Belarusian. Ukrainian is a descendent of Old East Slavic, a language spoken in the medieval state of Kievan Rus'. In the Grand Duchy of Lithuania, the language developed into Ruthenian, where it became an official language, before a process of Polonization began in the Polish–Lithuanian Commonwealth. By the 18th century, Ruthenian diverged into regional variants and the modern Ukrainian language developed in the territory of present-day Ukraine. In the Russian Empire, the Ukrainian language was later banned as a subject from schools and as a language of instruction as a result of Russification. However, through folk songs, itinerant musicians, and prominent authors, the language maintained a sufficient base in Western Ukraine. Linguistic development Theories Specifically Ukrainian developments that led to a gradual change of the Old East Slavic vowel system into the system found in modern Ukrainian began approximately in the 12th/13th century (that is, still at the time of the Kievan Rus') with a lengthening and raising of the Old East Slavic mid vowels e and o when followed by a consonant and a weak yer vowel that would eventually disappear completely, for example Old East Slavic котъ /kɔtə/ > Ukrainian кіт /kit/ 'cat' (via transitional stages such as /koˑtə̆/, /kuˑt(ə̆)/, /kyˑt/ or similar) or Old East Slavic печь /pʲɛtʃʲə/ > Ukrainian піч /pitʃ/ 'oven' (via transitional stages such as /pʲeˑtʃʲə̆/, /pʲiˑtʃʲ/ or similar). This raising and other phonological developments of the time, such as the merger of the Old East Slavic vowel phonemes и /i/ and ы /ɨ/ into the specifically Ukrainian phoneme /ɪ ~ e/, spelled with и (in the 13th/14th centuries), and the fricativisation of the Old East Slavic consonant г /g/, probably first to /ɣ/ (in the 13th century), with the result /ɦ/ in Modern Ukrainian, never happened in Russian, and only the fricativisation of Old East Slavic г /g/ occurred in Belarusian, where the result is /ɣ/. The