This script was a version of the Old Church Slavonic alphabet.

In the modern Church Slavonic alphabet, the 6th letter is typically shown as Єєе (one uppercase accompanied with two variants of lowercase).

The language was generally written in a Romanian Cyrillic alphabet (based on the Old Church Slavonic alphabet) before the 19th century.

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.