This article is about the Byzantine musical system of eight modes. For the book of liturgical texts set to those modes, see Octoechos (liturgy).
Oktōēchos (here transcribed "Octoechos"; Greek: ὁ Ὀκτώηχος[okˈtóixos]; from ὀκτώ "eight" and ἦχος "sound, mode" called echos; Slavonic: Осмогласие, Osmoglasie from о́смь "eight" and гласъ, Glagolitic: ⰳⰾⰰⱄⱏ, "voice, sound") is the name of the eight-mode system used for the composition of religious chant in Byzantine, Syrian, Coptic, Armenian, Georgian, Latin and Slavic churches since the Middle Ages. In a modified form the octoechos is still regarded as the foundation of the tradition of monodic Orthodox chant today.
The names ascribed to the eight tones differ in translations into Church Slavonic. Тhe Slavonic system counted the plagioi echoi as Glas 5, 6, 7, and 8. For reference, these differences are shown here together with the Ancient Greek names of the octave species according to the Hagiopolites and to the chant treatises and tonaries of Carolingian theorists. Please note that 15th-century composers like Manuel Chrysaphes, Lampadarios at the Court of Palaiologan Constantinople, exchanged the Phrygian with the Lydian. The Armenian names and their temporal cycles are represented in the article about the hymn books octoechos and parakletike.
^The female form ἡ Ὀκτώηχος exists as well, but means the book octoechos.
^ abAccording to the first paragraph of the Hagiopolites, John of Damascus is supposed to be the author of the 9th-century treatise: Raasted, Jørgen, ed. (1983). The Hagiopolites: A Byzantine Treatise on Musical Theory. Cahiers de l'Institut du Moyen-Âge Grec et Latin 45. Copenhagen: Paludan.