Feel free to download this Organ Accompaniment Booklet for Trinity Sunday (Second Vespers). Notice how the modes progress by number. Psalm 1 is mode 1; Psalm 2 is mode 2; Psalm 3 is mode 3; Psalm 4 is mode 4; Psalm 5 is mode 5. I am told by an expert that other feasts (such as Corpus Christi) are likewise organized by mode, and it’s called a “numerical office.”
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