“We decided to entrust this work to learned men of our selection. They very carefully collated all their work with the ancient codices in Our Vatican Library and with reliable, preserved or emended codices from elsewhere. Besides this, these men consulted the works of ancient and approved authors concerning the same sacred rites; and thus they have restored the Missal itself to the original form and rite of the holy Fathers.”
— Pope St. Pius V (Quo Primum, 1570)