And when you mix/match them, remember that the modes are keys. Especially if you are using organ accompaniment, use the transposer to dove-tail the Kyrie to the Gloria, and also to put each piece in the best singing range for your particular congregation. Gregorian chant solfege is a movable Do system, and simply has the 1/2-steps at different places. Even though there are "C" and "F" clefs in the chant, that does NOT correspond to the organ keyboard.

Thanks, but how are they grouped together? Are they grouped by someone by different sources, regions--? I know this donen't make any differences whether you know the answer or not in your singing chants, but it's good to be a more educated musician to help others (as well as for me) to understand chants.

The sets of pieces for the Ordinary, Kyrie, Gloria, Sanctus, and Agnus Dei (Credos are separate) have some tradition behind them; they were collected by Solesmes and published in their Kyriale, with the annotation that they were somewhat optional. Thus Mass I is almost uniformly found for the Easter season in the tradition, and Mass IX for feasts of the Blessed Virgin; Mass XI is very often found for ordinary Sundays, but not uniformly. Some traditions designate sets as for apostles, martyrs, confessors, etc.; others do it by the degree of the feast. All of this means that, other things being equal, one should keep to the designations in general, but one also has the option to do it otherwise. In the case of my own choir, we sing chant ordinaries only on ordinary Sundays (we sing a polyphonic Mass on feast days), so, in order that our congregation sing a wider variety of Masses, I have included Masses IV and VIII in the round off chants sung on ordinary Sundays.