As a result of earlier alternations between short and long vowels in roots in Proto-Indo-European, Proto-Balto-Slavic and Proto-Slavic times, and of the fronting of vowels after palatalized consonants, the following vowel alternations are attested in OCS: /ь/ : /i/; /ъ/ : /y/ : /u/; /e/ : /ě/ : /i/; /o/ : /a/; /o/ : /e/; /ě/ : /a/; /ъ/ : /ь/; /y/ : /i/; /ě/ : /i/; /y/ : /ę/.
As a result of earlier alternations between short and long vowels in roots in Proto-Indo-European Proto-Balto-Slavic and Proto-Slavic times and of the fronting of vowels after palatalized consonants the following vowel alternations are attested in OCS /ь/  /i/; /ъ/  /y/  /u/; /e/  /ě/  /i/; /o/  /a/; /o/  /e/; /ě/  /a/; /ъ/  /ь/; /y/  /i/; /ě/  /i/; /y/  /ę/