Papadic Octoechos
Oktōēchos (here transcribed "Octoechos"; Greek: ὁ Ὀκτώηχος, pronounced in Constantinopolitan: Ancient Greek pronunciation: [oxˈtoixos]; from ὀκτώ "eight" and ἦχος "sound, mode" called echos; Slavonic: Осмогласие, Osmoglasie from о́смь "eight" and гласъ "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 Orthodox chant today.