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According to tradition, Glagolitic was invented by the Byzantine Christian missionaries and brothers Cyril and Methodius in the 860s, amid the Christianization of the Slavs. Glagolitic appears to be older, predating the introduction of Christianity, only formalized by Cyril and expanded to cover non-Greek sounds. Cyrillic was created by the orders of Boris I of Bulgaria by Cyril's disciples, perhaps at the Preslav Literary School in the 890s.