The Cyrillic Alphabet was named for St. Cyril, although there is some dispute as to whether this is the alphabet he invented or not. Cyril was a Greek monk who, with Methodius, brought written language to Christian converts in the mid-9th century (c.860) in what is now Russia. The Cyrillic alphabet is closely based on the Greek alphabet, with about a dozen additional letters invented to represent Slavic sounds not found in Greek.
In Russia, Cyrillic was first written in the early Middle Ages in clear-cut, legible ustav (large letters). Later a succession of cursive forms developed. In the early eighteenth century, under Peter the Great, the forms of letters were simplified and regularized, with some appropriate only to Greek being removed. Further unnecessary letters were expunged in 1918, leaving the alphabet as it is todaystill in use in many Slavic Orthodox countries.
(Letters in parenthesis indicate the English transliteration of the Cyrillic letters.)