Chernorizets Hrabar is (as far as is known) the author of only one literary work, "On the Letters" (, O pismenĭhŭ, ), one of the most admired and popular works of literature written in Old Church Slavonic. The work was supposedly written sometime after the Preslav Ecclesiastical People's Council in 893, but before 921, and is the only known medieval literary work to quote the exact year of the invention of the Glagolitic alphabet (855). The work was partly based on Greek scholia and grammar treatises and expounded on the origin of the Glagolitic alphabet and Slavic Bible translation.