Old Church Slavonic
Old Church Slavonic (OCS) or Old Slavonic ( slə-VON-ik, slav-ON-) is the first Slavic literary language and the oldest extant written Slavonic language attested in literary sources. It belongs to the Eastern South Slavic subgroup of the South Slavic branch of the Slavic language family and is the ancestor of Church Slavonic, which remains the liturgical language of many Christian Orthodox churches.