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Oktoechos -LRB- here transcribed `` Octoechos '' ; Greek  : ο Οκτωηχος -LSB- okˈtoixos -RSB- ; from οκτω `` eight '' and ηχος `` sound , mode '' called echos ; Slavonic  : Осмогласие , Osmoglasie from осмь `` eight '' and гласъ , Glagolitic  : ⰳⰾⰰⱄⱏ , `` voice , sound '' -RRB- 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 .. Greek. Greek language. echos. echos. Slavonic. Church Slavonic. Glagolitic. Glagolitic. In a modified form the octoechos is still regarded as the foundation of the tradition of monodic chant in the Byzantine Rite today .. Byzantine Rite. Byzantine Rite