For the sake of example: let's say I'd like to point Ps. 51 to a Gregorian tone, and compose/point its antiphon (such as 'Be merciful, O Lord, for we have sinned' for Ash Wednesday). Would I arbitrarily assign a tone to the Psalm (I know I certainly could), or is there an established method or rule of assigning one of the nine tones to go with the text?
Since you mentioned the Psalm Miserere in the context of Ash Wednesday, I'll note that the famous Allegri setting of this psalm uses the mode 2 psalm tone. As indicated in program notes for a recent concert, the historical, liturgical setting was matins for Holy Week at the Sistine Chapel. So it depends.
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