Cyrillic is derived from the Greek uncial script, augmented by letters from the older Glagolitic alphabet, including some ligatures. These additional letters were used for Old Church Slavonic sounds not found in Greek. The script is named in honor of the two Byzantine brothers, Saints Cyril and Methodius, who created the Glagolitic alphabet earlier on. Modern scholars believe that Cyrillic was developed and formalized by early disciples of Cyril and Methodius.
In the early 18th century, the Cyrillic script used in Russia was heavily reformed by Peter the Great, who had recently returned from his Grand Embassy in western Europe. The new form of letters became closer to the Latin alphabet, several archaic letters were removed and several letters were personally designed by Peter the Great (such as Я which was inspired by Latin R). West European typography culture was also adopted.
Cyrillic script spread throughout the East Slavic and some South Slavic territories, being adopted for writing local languages, such as Old East Slavic. Its adaptation to local languages produced a number of Cyrillic alphabets, discussed hereafter.
Capital and lowercase letters were not distinguished in old manuscripts.
Yeri (Ы) was originally a ligature of Yer and I (Ъ + І = Ы). Iotation was indicated by ligatures formed with the letter І: Ꙗ (not ancestor of modern Ya, Я, which is derived from Ѧ), Ѥ, Ю (ligature of І and ОУ), Ѩ, Ѭ. Sometimes different letters were used interchangeably, for example И = І = Ї, as were typographical variants like О = Ѻ. There were also commonly used ligatures like ѠТ = Ѿ.