Cyril and his elder brother Methodius already have a distinguished reputation as theologians and linguists when the Byzantine emperor sends them as missionaries, in 863, to the Slavs of Moravia. The brothers are Greek but they know the Slavonic language spoken in their native region of Salonika. In Moravia they conduct church services in Slavonic. Naturally they wish to write down this liturgy, together with their own Slavonic translation of parts of the Bible. But there is no Slavonic script.

Like Ulfilas before them with Gothic, the brothers need to devise a new alphabet for their purpose.

Cyril and Methodius base their new letters loosely on Greek examples. The Slavonic alphabet is known today as cyrillic after the more forceful of the brothers - though in its surviving form it is probably devised by Cyril's followers in Bulgaria rather than the saint himself (whose original invention is more likely to be the now extinct glagolitic alphabet).

Nevertheless the remarkable fact is that cyrillic remains the script of all the Slav regions which adopt the Greek Orthodox faith - including Serbia, Bulgaria and above all Russia.