Oktōēchos (here transcribed "Octoechos"; Greek: ὁ Ὀκτώηχος ; 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. 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. The 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 (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. The Armenian names and their temporal cycles are represented in the article about the hymn books octoechos and parakletike.Southern Slavs use the Byzantine musical system and, nonetheless, use the variant numbering that is always found in Church Slavonic texts.