Is Byzantine Octoechos a musical system?
Dorian mode
The early Byzantine church developed a system of eight musical modes (the octoechos), which served as a model for medieval European chant theorists when they developed their own modal classification system starting in the 9th century. The success of the Western synthesis of this system with elements from the fourth book of "De institutione musica" of Boethius, created the false impression that the Byzantine octoechos was inherited directly from ancient Greece.
Neobyzantine Octoechos
Despite of this pure chromatic form, the "plagios devteros" in the "sticheraric" and "papadic melos" is the only echos of the Octoechos which combines as well and in a temporary change of genus (μεταβολή κατὰ γένος) two different "genera" within the two tetrachords of the protos octave, while the diatonic tetrachord lies between δ' and γ' (a triphonic construction πλα'—δ'—γ'), and has the melos of "echos tetartos" (as shown in the mixed example of its "parallage"). As could already studied in the "parallage of Ioannis Plousiadinos", the same chromatic tetrachord could be as well that of the protos (πλα'—δ') like in the older "enechemata" of "phthora nenano". The other mixed form, though rare, was the combination of phthora nenano on the protos parallage (α' κε—δ' πα') with a diatonic protos pentachord under it (πλ α' πα—α' κε).