The Kyrie Eleison:
52. After the Act of Penitence, the Kyrie is always begun, unless it has already been included as part of the Act of Penitence. Since it is a chant by which the faithful acclaim the Lord and implore his mercy, it is ordinarily done by all, that is, by the people and the choir or cantor having a part in it.
As a rule, each acclamation is sung or said twice, though it may be repeated several times, by reason of the character of the various languages, as well as of the artistry of the music or of other circumstances. When the Kyrie is sung as a part of the Act of Penitence, a trope may precede each acclamation.
The Gloria:
53. The Gloria is a very ancient and venerable hymn in which the Church, gathered together in the Holy Spirit, glorifies and entreats God the Father and the Lamb. The text of this hymn may not be replaced by any other text. The Gloria is intoned by the priest or, if appropriate, by a cantor or by the choir; but it is sung either by everyone together, or by the people alternately with the choir, or by the choir alone. If not sung, it is to be recited either by all together or by two parts of the congregation responding one to the other.
It is sung or said on Sundays outside the Seasons of Advent and Lent, on solemnities and feasts, and at special celebrations of a more solemn character.
Gloria (even on a Sunday during Advent and Lent; choice of musical settings)
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