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The current usage of Byzantine Chant is built upon eight modes (tones), each mode with its own specific tonality.
The modes change sequentially from week to week, starting the Monday after the [[Sunday of St. Thomas]], with the First Mode (1).
At Great Vespers, typically the Kekragarion (Lord I Have Cried & Let My Prayer Arise), the following verses, the Stichera of the Resurrection and the Aposticha of the Resurrection all correspond to the mode of the week. Great Vespers is also dismissed with the Apolytikion (Troparion) of the Resurrection in the appropriate mode of the week. Orthros and the Divine Liturgy on Sunday likewise follow the mode of the week with certain hymns and readings. Services starting with Sunday Vespers to the end of the week all rely on whatever daily commemoration and festal commemoration is called for in the liturgical rubrics.
Within [[Bright Week]] itself, the mode changes each day, thus:
:Bright Sunday – First Mode (1),
The Grave Mode (7) was chosen as the mode to be left out due to its heavier sound, considered least appropriate for the festal period among the eight modes. Since [[Pentecost]] falls on the Sunday when the grave mode would have been used in the normal sequence, the mode is once again skipped and the hymns of Pentecost are used. The sequence resumes the following week with Plagal of the Fourth Mode (8).
In the time between a [[Great Feasts|great feast]] and its [[Leavetaking|leave-taking]], for example, during the week following Pentecost, the hymns of the feast are chanted rather than the hymns pertaining to the mode of the week.