Early Cyrillic alphabet in the context of "Preslav Literary School"

The Slavs were Christianized in waves from the 7th to 12th century, though the process of replacing old Slavic religious practices began as early as the 6th century. Generally speaking, the monarchs of the South Slavs adopted Christianity in the 9th century, the East Slavs in the 10th, and the West Slavs between the 9th and 12th century. Saints Cyril and Methodius (fl. 860–885) are attributed as "Apostles to the Slavs", having introduced the Byzantine-Slavic rite (Old Slavonic liturgy) and Glagolitic alphabet, the oldest known Slavic alphabet and basis for the Early Cyrillic alphabet.

The simultaneous missionary efforts to convert the Slavs by what would later become known as the Catholic Church of Rome and the Eastern Orthodox Church of Constantinople led to a 'second point of contention between Rome and Constantinople', especially in Bulgaria (9th–10th century). This was one of many events that preceded the East–West Schism of 1054 and led to the eventual split between the Greek East and Latin West. The Slavs thus became divided between Eastern Orthodoxy and Roman Catholicism. Closely connected to the competing missionary efforts of the Roman Church and the Byzantine Church was the spread of the Latin and Cyrillic scripts in Eastern Europe. The majority of Orthodox Slavs adopted Cyrillic, while most Catholic Slavs adopted the Latin, but there were many exceptions to this general rule. In areas where both Churches were proselytising to pagan Europeans, such as the Grand Duchy of Lithuania, the Croatian Duchy and the Principality of Serbia, mixtures of languages, scripts and alphabets emerged, and the lines between Latin Catholic (Latinitas) and Cyrillic Orthodox literacy (Slavia Orthodoxa) were blurred.

The Glagolitic script (/ˌɡlæɡəˈlɪtɪk/ GLAG-ə-LIT-ik, ⰳⰾⰰⰳⱁⰾⰻⱌⰰ, glagolitsa) 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 after an invitation from Rastislav of Moravia to spread Christianity there. After the deaths of Cyril and Methodius, their disciples were expelled from Moravia, and they moved to the First Bulgarian Empire instead. The Early Cyrillic alphabet, which was developed gradually in the Preslav Literary School by scribes who incorporated some Glagolitic letters when writing in the Greek alphabet, gradually replaced Glagolitic in that region. Glagolitic remained in use alongside the Latin script in the Kingdom of Croatia and alongside Cyrillic until the 14th century in the Second Bulgarian Empire and the Serbian Empire; in later periods, it was used mainly for cryptographic purposes.

Glagolitic also spread to the Kievan Rus' and the Kingdom of Bohemia. Although its use declined there in the 12th century, some manuscripts in the territory of the former retained Glagolitic inclusions for centuries. It had also spread to Duklja and Zachlumia in the Western Balkans, from where it reached the March of Verona. There, the Investiture Controversy afforded it refuge from the opposition of Latinizing prelates and allowed it to entrench itself in Istria, from which place it spread to nearby lands. It survived there and as far south as Dalmatia without interruption into the 20th century for Church Slavonic in addition to its use as a secular script in parts of its range, which at times extended into Bosnia, Slavonia, and Carniola, in addition to 14th–15th century exclaves in Prague and Kraków, and a 16th-century exclave in Putna.

The Ukrainian alphabet (Ukrainian: абе́тка, áзбука, алфа́ві́т, or альфабе́т [1928–1933 spelling and before 1933], romanized: abétka, ázbuka, alfávít, or alʹfabét) is the set of letters used to write Ukrainian, which is the official language of Ukraine. It is one of several national variations of the Cyrillic script. It comes from the Cyrillic script, which was devised in the 9th century for the first Slavic literary language, called Old Slavonic. In the 10th century, Cyrillic script became used in Kievan Rus' to write Old East Slavic, from which the Belarusian, Russian, Rusyn, and Ukrainian alphabets later evolved. The modern Ukrainian alphabet has 33 letters in total: 21 consonants, 1 semivowel, 10 vowels and 1 palatalization sign. Sometimes the apostrophe (') is also included, which has a phonetic meaning and is a mandatory sign in writing, but is not considered as a letter and is not included in the alphabet.

In Ukrainian, it is called українська абетка (tr. ukrainska abetka, IPA: [ʊkrɐˈjinʲsʲkɐ ɐˈbɛtkɐ]), from the initial letters а (tr. a) and б (tr. b); алфавіт (tr. alfavit); or, archaically, азбука (tr. azbuka), from the acrophonic early Cyrillic letter names азъ (tr. az) and буки (tr. buki).

In Slavic languages, iotation (/joʊˈteɪʃən/ yoh-TAY-shən or /ˌaɪ.oʊˈteɪʃən/ EYE-oh-TAY-shən) is a form of palatalization that occurs when a consonant comes into contact with the palatal approximant /j/ from the succeeding phoneme. The /j/ is represented by iota (ι) in the early Cyrillic alphabet and the Greek alphabet on which it is based. For example, ni in English onion has the sound of iotated n. Iotation is a phenomenon distinct from Slavic first palatalization in which only the front vowels are involved, but the final result is similar.

Yat or jat (Ѣ ѣ; italics: Ѣ ѣ) is the thirty-second letter of the old Cyrillic alphabet.

There is also another version of yat, the iotated yat (majuscule: ⟨Ꙓ⟩, minuscule: ⟨ꙓ⟩), which is a Cyrillic character combining a decimal I and a yat. There was no numerical value for this letter and it was not in the Glagolitic alphabet. It is encoded in Unicode 5.1 at positions U+A652 and U+A653.

Little yus (Ѧ) and big yus (Ѫ), or jus, are two letters of the Cyrillic script representing two Common Slavonic nasal vowels in the early Cyrillic and Glagolitic alphabets. Each can occur in iotated form (Ѩ, Ѭ), formed as ligatures with the decimal i (І). Other yus letters are closed little yus (Ꙙ), iotated closed little yus (Ꙝ) and blended yus (Ꙛ).

Phonetically, little yus represents a nasalized front vowel, possibly [ɛ̃] (like the French ‘in’ in “cinq” or Polish 'ę' in “kęs”), while big yus represents a nasalized back vowel, such as IPA [ɔ̃] (like the French ‘on’ in “bombon” or Polish 'ą' in “kąt”). This is also suggested by the appearance of each as a 'stacked' digraph of 'Am' and 'Om' respectively.