Theognostus the Grammarian (‹See Tfd›Greek: Θεόγνωστος ὁ Γραμματικός; Latin: Theognostus Grammaticus) was a Byzantine grammarian of the 9th century and the author of Περὶ ὀρθογραφίας (On Orthography), also known as the Κανόνες (Canons), a handbook on spelling.[1] The work, which is based on the works of the earlier grammarians Cyril and Herodian, consists of a series of rules designed to help Byzantine writers use the correct ancient spellings of words whose pronunciation had changed in mediaeval Greek.[2] It is dedicated to the emperor Leo V.[3] Theognostus also wrote a lost work on the rebellion of Euphemius in Sicily in 826–827.[4]