The Early Cyrillic alphabet, also called classical Cyrillic or paleo-Cyrillic, is a writing system that was developed in Medieval Bulgaria in the Preslav Literary School during the late 9th century. The modern Cyrillic script is used for some Slavic languages (such as Bulgarian, Macedonian, Serbian, Russian, Ukrainian), and for East European and Asian languages that have experienced a great amount of Russian cultural influence.
Taken from here: https://www.unicode.org/notes/tn41/tn41-1.pdf
