On 22nd November, 1903, the Feast of St. Cecilia, St. Pius X issued his Motu Proprio Tra le Sollecitudini on Sacred Music:
22. It is not lawful to keep the priest at the altar waiting on account of the chant or the music for a length of time not allowed by the liturgy. According to the ecclesiastical prescriptions the Sanctus of the Mass should be over before the elevation, and therefore the priest must here have regard for the singers. The Gloria and the Credo ought, according to the Gregorian tradition, to be relatively short.
In a letter regarding Tra le Sollecitudini addressed to the Cardinal Vicar of Rome, dated 8th December, 1903, St. Pius X wrote:
Finally, We desire that sacred music be cultivated with special care and in the proper way in all the seminaries and ecclesiastical colleges of Rome, in which such a large and choice body of young clerics from all parts of the world are being educated in the sacred sciences and in the ecclesiastical spirit. We know, and We are greatly comforted by the knowledge, that in some institutions sacred music is in such a flourishing condition that it may serve as a model for others. But there are some seminaries and colleges which leave much to be desired owing to the carelessness of the superiors, or the want of capacity and the imperfect taste of the persons to whom the teaching of chant and the direction of sacred music is entrusted. You, Lord Cardinal, will be good enough to provide a remedy for this also with solicitude, by insisting especially that Gregorian Chant, according to the prescriptions of the Council of Trent and of innumerable other councils, provincial and diocesan in all parts of the world, be studied with particular diligence, and be as a rule preferred in the public and private functions of the institute. It is true that in other times Gregorian Chant was known to most people only through books which were incorrect, vitiated and curtailed. But the accurate and prolonged study that has been given to it by illustrious men who have done a great service to sacred art has changed the face of things. Gregorian Chant restored in such a satisfactory way to its early purity, as it was handed down by the fathers and is found in the codices of the various churches, is sweet, soft, easy to learn and of a beauty so fresh and full of surprises that wherever it has been introduced it has never failed to excite real enthusiasm in the youthful singers. Now, when delights enters into the fulfillment of duty, everything is done with greater alacrity and with more lasting fruit. It is Our will, therefore, that in all seminaries and colleges in this fostering city there be introduced once more the most ancient Roman chant which used to resound in our churches and basilicas and which formed the delight of past generations in the fairest days of Christian piety. And as in former times the chant was spread abroad over the whole Western Church from Rome, so We desire that Our young clerics, educated under Our own eyes, may carry it with them and diffuse it again in their own dioceses when they return thither as priests to work for the glory of God. We are overjoyed to be able to give these regulations at a time when We are about to celebrate the 15th centenary of the death of the glorious and incomparable Pontiff St. Gregory the Great, to whom an ecclesiastical tradition dating back many centuries has attributed the composition of these sacred melodies and from whom they have derived their name. Let Our dearly-beloved youths exercise themselves in them, for it will be sweet to us to hear them when, as We have been told will be the case, they will assemble at the coming centenary celebrations round the tomb of the Holy Pontiff in the Vatican Basilica during the Sacred Liturgy which, please God, will be celebrated by Us on that auspicious occasion.