A preparatory drawing directly on a ground, which is subsequently covered with paint. Such drawings are often executed with charcoal, chalk, pencil, or paint. Depending on the artist and the function of the picture, underdrawings can range from quick sketched outlines to quite detailed renderings. Until recently, this type of drawing was visible to the naked eye primarily in unfinished works or when thin paint layers become more transparent over time. The technique of infrared reflectography has made it possible to view some underdrawings through the paint layer. The study of the changes made between the drawing and the painted image allows us to observe the creative process of the painted in greater detail than before.

Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 82.