44 It is the duty of all those to whom Christ the Lord has entrusted the task of guarding and dispensing the Church's riches to preserve this precious treasure of Gregorian chant diligently and to impart it generously to the Christian people. Hence what Our predecessors, St. Pius X, who is rightly called the renewer of Gregorian chant, and Pius XI have wisely ordained and taught, We also, in view of the outstanding qualities which genuine Gregorian chant possesses, will and prescribe that this be done. In the performance of the sacred liturgical rites this same Gregorian chant should be most widely used and great care should be taken that it should be performed properly, worthily and reverently. And if, because of recently instituted feast days, new Gregorian melodies must be composed, this should be done by true masters of the art. It should be done in such a way that these new compositions obey the laws proper to genuine Gregorian chant and are in worthy harmony with the older melodies in their virtue and purity.
Source: Musicae Sacrae (Vatican.va)