The Glagolitic script is the oldest known Slavic alphabet. It is generally agreed that it was created in the 9th century for the purpose of translating liturgical texts into Old Church Slavonic by Saint Cyril, a monk from Thessalonica. He and his brother Saint Methodius were sent by the Byzantine Emperor Michael III in 863 to Great Moravia to spread Christianity there. After the deaths of Cyril and Methodius, their disciples were expelled and they moved to the First Bulgarian Empire instead. The Cyrillic alphabet, which developed gradually in the Preslav Literary School by Greek alphabet scribes who incorporated some Glagolitic letters, gradually replaced Glagolitic in that region. Glagolitic remained in use alongside Latin in the Kingdom of Croatia and alongside Cyrillic until the 14th century in the Second Bulgarian Empire and the Serbian Empire, and later mainly for cryptographic purposes.

An alphabet is a standard set of letters written to represent particular sounds in a spoken language. Specifically, letters correspond to phonemes, the categories of sounds that can distinguish one word from another in a given language. Not all writing systems represent language in this way: a syllabary assigns symbols to spoken syllables, while logographies assign symbols to words, morphemes, or other semantic units.