writing system developed in the 9th–10th century AD for Slavic-speaking peoples of the Eastern Orthodox faith; it is the alphabet currently used for Russian and other languages of the republics that once formed the Soviet Union and for Bulgarian and Serbian. Based on the medieval Greek uncial script, the Cyrillic alphabet was probably invented by later followers of the 9th-century “apostles to the Slavs,” St. Cyril (or Constantine), for whom it was named, and St. Methodius. As the Slavic languages were richer in sounds than Greek, 43 letters were originally provided to represent them; the added letters were modifications or combinations of Greek letters, or (in the case of the Cyrillic letters for ts, sh, and ch) they were based on Hebrew. The earliest literature written in Cyrillic was a translation of the Bible and various church texts.

The modern Cyrillic alphabets—Russian, Ukrainian, Bulgarian, and Serbian—have been modified somewhat from the original, generally by the loss of some superfluous letters. Modern Russian has 32 letters (33, with inclusion of the soft sign—not strictly a letter), Bulgarian 30, Serbian 30, and Ukrainian 32 (33). Modern Russian Cyrillic has also been adapted to many non-Slavic languages, sometimes with the addition of special letters.