Papers presented at the 17th Symposium for the Study of Underdrawing and Technology in Painting, held in Leuven on 22-24 October 2009. The earliest known copy (1443) of Van der Weyden's masterpiece was extensively studied in preparation for an exhibition at the Museum M in Leuven. It underwent conservation treatment at the Royal Institute for Cultural Heritage in Brussels, leaving Saint Peter's at Leuven for the first time since 1960. The study involved an art historical assessment and full technical examination of the triptych, whose authorship is still elusive. The paper discusses several art historical and technical issues, such as its unity of style and technique and its relationship to the prototype.