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.
TheoFonsAd fontes fidei