This Requiem is known in five sources; two of them mention no composer, two attribute it to Antoine de Févin, one, the Occo Codex, to Antoine Divitis. It is recorded here in the version transmitted by an early sixteenth-century manuscript, the Occo Codex, a sumptuous, richly illuminated volume. The book was originally intended for use in worship at one of the oldest churches in Amsterdam, built in the fourteenth century on the site of a miracle which played a fundamental role in constituting the religious identity of Amsterdam. Occo was the name of the rich merchant who financed the production of the manuscript. It contains some fifteen masses by great composers of the fifteenth century. Composed at the very end of the fifteenth century and has not yet received the full measure of attention it deserves. It is a luminous work. The plainchant melodies, constantly present, sculpted by the living flesh of the polyphonic texture, blaze, shine and reveal the inner energy that, cutting through the centuries, fashions the deployment of time. Antoine Divitis and Antoine de Févin belonged to the last generation of those men who, at the end of the so-called Gothic era, brought to its highest degree of sophistication the science of numbers applied to the art of sounds. Not until Johann Sebastian Bach and, later, the serial combinations of the twentieth century will we meet once more the fascination that comes from the contemplation in sound of the laws of numbers and then it will be in a quite different context. For these men of the fourteenth and fifteenth centuries were not only composers; they were first of all members of a choir all of them priests and canons attached to the service of a ritual, and through the effect of their consecrated voices they manifested the presence of a tradition, not as an outmoded catalogue of conventional gestures and words, but as a living energy, an immanent flame that illumines the awareness of being present at a ceremony of cosmic, telluric and eschatological organisation: the organisation of liturgical time.