Liber usualis explains how to sing the tones of prophecy and epistle.
But I would like to know when use these tones in the Tridentine Mass?
For example in both cases (as a prophecy and an epistle) there is presented the title of a lesson from the book of Isaiah.
So, should always the lesson from the Old Testament be sung on the prophecy tone ? Or if there is only one lesson immediately before Gospel should it be always sung on the epistle tone?
If there are more lessons should all these lessons (except the last one before Gospel) be sung on the prophecy tone?
Do you know any rules regarding this problem (I cannot find any answer in Liber Usualis and Graduale Romanum)?
Aga, in my experience with the EF Mass where the tones for the readings have been used, the practice has been to use the epistle tone for the lesson before the Gospel.* The only times the prophecy tone was used was, as you said, when there were more lessons preceding the last one before the Gospel (Easter Vigil, Ember days, etc.).
I think the overarching rule would be: if the priest or subdeacon knows the formula, he ought to use it consistently. :¬)
*The option to sing the reading before the Gospel on a straight tone (recto tono) has been more commonly employed at EF Masses I have attended.
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