Although it has always been plain to see that the Milanese and Roman Mass shared many texts, that the melodies were also shared has gone largely unnoticed, or at any rate undemonstrated, except in the special case of the few offertory chants with wider concordances in Franco-Roman, Milanese and Visigothic books. This article takes up the particular case of the Roman and Milanese entrance antiphons: first, the circumstances of the importation of the Roman introits into the Ambrosian Mass; and, second, the precise relationship of the Ambrosian and Franco-Roman (Gregorian) melodies. It has long been understood that chants of the Old Roman repertory provide a firm basis for an understanding of the changes, inevitable over time, in an orally transmitted repertory. It emerges that the Ambrosian melodies, transcribed in neumes in about the middle of the eleventh century, offer a second opportunity for a sondage. This other, unsuspected, version of the chants, miraculously preserved and stabilised north of the Alps in the ninth and tenth centuries also allows for convincing demonstrations of the musical procedures employed in the elaboration of the melodies and in their adaptation to different texts. And not least, the isolation of what is shared between versions notated at a distance of centuries give us the basis of an objective estimation of the effectiveness of musical memory in a musical culture that did not rely on notation.