Oktōēchos (here transcribed "Octoechos"; Greek: ὁ Ὀκτώηχος Ancient Greek pronunciation: [okˈtóixos];[1] from ὀκτώ "eight" and ἦχος "sound, mode" called echos; Slavonic: Осмогласие, Osmoglasie from о́смь "eight" and гласъ, Glagolitic: ⰳⰾⰰⱄⱏ, "voice, sound") is the eight-mode system used for the composition of religious chant in Byzantine, Syriac, Armenian, Georgian, Latin and Slavic churches since the Middle Ages.
For reference, these differences are shown here together with the Ancient Greek names of the octave species according to the Hagiopolites[2] (see Hagiopolitan Octoechos) and to the chant treatises and tonaries of Carolingian theorists.
Fifteenth-century composers like Manuel Chrysaphes, Lampadarios at the Court of Palaiologan Constantinople exchanged the Phrygian with the Lydian.
Southern Slavs use the Byzantine musical system and, nonetheless, use the variant numbering that is always found in Church Slavonic texts.
[3] According to three main periods, which divides the history of the eight-mode system, the former article has been split chronologically: Byzantine Chant performance practice has been computationally compared to the theory by Chrysanthos.