The Cyrillic I is a letter used in almost all Cyrillic alphabets with the exception of Belarusian.
И, from Alexandre Benois' 1904 азбука.
In early Russian typewriters like this one, there was no key for the digit 1, so the dotted І was used instead. Following the Russian alphabet reform of 1918, a 1 key was added.
Numerous Cyrillic alphabets are based on the Cyrillic script. The early Cyrillic alphabet was developed in the 9th century AD and replaced the earlier Glagolitic script developed by the theologians Cyril and Methodius. It is the basis of alphabets used in various languages, past and present, Slavic origin, and non-Slavic languages influenced by Russian. As of 2011, around 252 million people in Eurasia use it as the official alphabet for their national languages. About half of them are in Russia. Cyrillic is one of the most-used writing systems in the world. The creator is Saint Clement of Ohrid from the Preslav literary school in the First Bulgarian Empire.
The first lines of the Book of Matthew in Karelian using the Cyrillic script, 1820