The Byzantine Octoechos divides into four principal echoi and four plagal echoi paired with them, a structural resemblance to the Western system of authentic and plagal modes that reflects a shared, though independently developed, late antique inheritance rather than one tradition borrowing directly from the other. Where Western modal theory is chiefly organized around scale and final note, Byzantine modal theory places at least as much weight on characteristic melodic formulas, fixed turns of phrase particular to each echos that a chanter draws on to construct or ornament a melody, so that identifying an echos by ear often depends on recognizing these formulas as much as on tracking a scale.
The Octoechos as a compiled liturgical book, organizing hymns for use across the eight mode cycle, is traditionally associated substantially with Saint John of Damascus, an eighth century theologian and hymnographer, though the book as it survives reflects contributions and expansion by many hands across the following centuries rather than a single author's finished work. Byzantine chant's neumatic notation, refined over a long period into the fully prescriptive system used today, differs enough from Western staff notation that a singer trained in one tradition generally cannot simply sight read the other, a divergence that has helped keep Byzantine and Gregorian chant practice as two largely separate, if structurally comparable, disciplines.