Main article: Early Cyrillic Alphabet
he Cyrillic script was created in the First Bulgarian Empire and is derived from the Greek uncial script letters, augmented by ligatures and consonants from the older Glagolitic alphabet for sounds not found in Greek. Tradition holds that Cyrillic and Glagolitic were formalized either by the two Greek brothers born in Thessaloniki, Saints Cyril and Methodius who brought Christianity to the southern Slavs, or by their disciples. Paul Cubberley posits that while Cyril may have codified and expanded Glagolitic, it was his students in the First Bulgarian Empire that developed Cyrillic from the Greek letters in the 890s as a more suitable script for church books. Later Cyrillic spread among other Slavic peoples: Russians, Serbs and others, as well as among non-Slavic Vlachs and Moldavians.
Cyrillic and Glagolitic were used for the Church Slavonic language, especially the Old Church Slavonic variant. Hence expressions such as “И is the tenth Cyrillic letter” typically refer to the order of the Church Slavonic alphabet; not every Cyrillic alphabet uses every letter available in the script.
The Cyrillic script came to dominate Glagolitic in the 12th century. The literature produced in the Old Bulgarian language soon spread north and became the lingua franca of the Balkans and Eastern Europe, where it came to also be known as Old Church Slavonic. The alphabet used for the modern Church Slavonic language in Eastern Orthodox and Eastern Catholic rites still resembles early Cyrillic. However, over the course of the following millennium, Cyrillic adapted to changes in spoken language, developed regional variations to suit the features of national languages, and was subjected to academic reform and political decrees. Today, dozens of languages in the Balkans, Eastern Europe, and northern Eurasia are written in Cyrillic alphabets.