* Greek: The basic structure and most of the letters are derived from the Greek alphabet.
* Glagolitic: This is another early Slavic alphabet, and some Cyrillic letters were adapted from it.
* Old Church Slavonic: This was the liturgical language used by early Slavs, and many of the sounds in Cyrillic reflect the sounds of this language.
It's important to note that Cyrillic didn't simply borrow letters directly from these languages. The creators of Cyrillic adapted and modified the letters to better fit the sounds of the Slavic languages.