The Cyrillic script, sometimes called the Slavonic script or Slavic script, is a writing system used to write Russian, Ukrainian, Belarusian, Rusyn, Bulgarian, Macedonian and for most South Slavic languages. It was developed in the Bulgarian empire during the reign of Tsar Simeon I. The Soviet Union made many non-Slavic languages in the Caucasus, Siberia, Central Asia and in northern Russia to be written in Cyrillic. [source?]