Moravia experienced significant cultural development under King Rastislav, with the arrival in 863 of the mission of Saints Cyril and Methodius. After his request for missionaries had been refused in Rome, Rastislav asked the Byzantine emperor to send a "teacher" (učiteľ) to introduce literacy and a legal system (pravьda) to Great Moravia. The request was granted. The missionary brothers Cyril and Methodius introduced a system of writing (the Glagolitic alphabet) and Slavonic liturgy, the latter eventually formally approved by Pope Adrian II. The Glagolitic script was probably invented by Cyril himself and the language he used for his translations of holy scripts and his original literary creation was based on the Slavic dialect he and his brother Methodius knew from their native Thessaloniki. The language, termed Old Church Slavonic, became the official written language for the Bulgars who refer to it as Old Bulgarian. Old Church Slavonic, therefore, differed somewhat from the local Slavic dialect of Great Moravia which was the ancestral idiom to the later dialects spoken in Moravia and western Slovakia.