Thanks to improvements in the quality of infrared reflectograms, it is possible to try to compare the underdrawings of different paintings by precisely characterizing them. The author tries to elaborate a system of underdrawing description for the Hague <i> Pietà</i>, traditionally attributed to Rogier van der Weyden. At this time this effort can be regarded as having a rather theoretical value which may nevertheless lead to consider some aspects of stylistic comparisons under a new light. -- AATA