Orthodox worship assigns a specific echos to each week in an eight week rotation tied to the church calendar, so that the resurrectional hymns sung at Saturday evening Vespers and Sunday Matins change mode from week to week in a fixed, predictable sequence rather than by the choir's or priest's own choice, a system that gives the Octoechos its name as much a description of a liturgical schedule as of a set of eight melodic types. This same eight mode framework was carried, with substantial local adaptation, into the various national and linguistic branches of Eastern Orthodox and Eastern Catholic chant, so that Slavic traditions such as Russian Znamenny chant, Georgian chant and Arabic speaking Orthodox chant, the latter drawing heavily on Arabic maqam theory, all organize their repertoires around some version of the same eight mode structure while developing quite distinct melodic vocabularies within it.
A chanter trained in one of these regional traditions can typically recognize the underlying eight mode logic in another even where the actual melodies sound unfamiliar, evidence of how deeply the Byzantine modal framework shaped Eastern Christian liturgical music well beyond the Greek speaking heartland where it originated. The Octoechos remains a core liturgical book in active use today, printed and reprinted for parish and monastic use across the Orthodox world exactly as a practical schedule of which hymns and which mode belong to which week.