Oktōēchos is the name of the eight mode system used for the composition of religious chant in most Christian churches during the Middle Ages. In a modified form the octoechos is still regarded as the foundation of the tradition of monodic Orthodox chant today.
Intonation according to Erotapokriseis and standard intonation of echos protos: "You descend 4 steps [φοναὶ] from the echos protos [kyrios protos/authentic protus] and you will find again the plagios protos, this way"
Intonation according to Erotapokriseis and standard intonation of echos devteros: "You do the same way in echos devteros. If you descend 4 steps to find its plagios, i.e. πλ β', thus"
The earliest tonary: the fragment of Saint-Riquier (F-Pn lat. Ms. 13159, fol. 167r)
The intonation formulas for the 8 tones according to the Aquitanian tonary of Adémar de Chabannes (F-Pn lat. Ms. 909, fol. 251r-254r)
Echos is the name in Byzantine music theory for a mode within the eight-mode system (oktoechos), each of them ruling several melody types, and it is used in the melodic and rhythmic composition of Byzantine chant, differentiated according to the chant genre and according to the performance style. It is akin to a Western medieval tonus, an Andalusian tab', an Arab naġam, or a Persian parde.
Intonation according to Erotapokriseis and standard intonation of echos protos: "You descend 4 steps [φοναὶ] from the echos protos [kyrios protos/authentic protus] and you will find again the plagios protos, this way"
Intonation according to Erotapokriseis and standard intonation of echos devteros: "You do the same way in echos devteros. If you descend 4 steps to find its plagios, i.e. πλ β', thus"
"Parallage of John Koukouzeles": The four peripheral wheels for the Octoechos (top left: protos echoi; top right: devteros echoi; bottom left: tritos echoi; bottom right: tetartos echoi) and the tetraphonic tone system and its transpositions in the center—Koukouzelian wheel in an 18th-century manuscript (manuscript of the private collection by Demetrios Kontogiorges)
Intonations (ἐνηχήματα) for the echoi plagios devteros, varys, and plagios tetartos listed in a 17th-century papadike treatise—the intonations are followed by chant incipits (London, British Library, Harley 5544, fol. 8r)