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, Syriac, 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 chant in the Byzantine Rite 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.
Byzantine Chant performance practice has been computationally compared to the theory by Chrysanthos. The analysis of 94 Byzantine Chants performed by 4 singers showed a tendency of the singers to level theoretic particularities of the echos that stand out of the general norm in the octoechos. In practice, smaller scale degree steps (67-133 cents) appear to be increased and the highest scale step of 333 cents appears to be decreased compared to theory. In practice, the first four scale notes in decreasing order of prominence I, III, II, IV are more prominent than the V., VI., and the VII..
^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.
^Panteli, Maria; Purwins, Hendrik (2013). "A Quantitative Comparison of Chrysanthine Theory and Performance Practice of Scale Tuning, Steps, and Prominence of the Octoechos in Byzantine Chant". Journal of New Music Research42 (3): 205–221. doi:10.1080/09298215.2013.827215.