Thanks to the technique of infra-red reflectography, we can see the preparatory drawing which formed the structure and composition of these paintings beneath the paint layers. In some cases relating to fifteenth- and sixteenth-century painting, the reflectographs are the only means of seeing drawings by artists whose works on paper have not survived. It has thus been possible to discover the graphic style of artists such as Robert Campin, Memling and Pedro Berruguete, whose drawings would otherwise be completely or almost totally unknown to us. Drawing is of course a highly personal and unique mode of artistic expression for the painter and can thus be seen as a document as well as a piece of artistic evidence of primary importance for the study of the work of certain artists and the attribution of their works.