St Gereon’s armour and clothing are clearly underdrawn. The use of a quill as drawing instrument is revealed in the lay-in of the cloak by the often blunt, angular or sometimes even split beginnings to the strokes, the mostly short length of the strokes, and the rhythmic alternation of thin and thick lines in the zigzag hatching.
In large parts, the lines, with their short and sometimes parallel strokes, which sometimes intersect and sometimes do not even meet, create the impression that the artist was searching for form in some places, while being consciously sketchy in others. This is also true of the drawing of the protective clothing of the upper body, which below a V-neck with occasionally indicated jewellery, seems to imitate the musculature of the chest, and also depicts a chain running diagonally across the body. The shield-like plates which replace the usual circular besagues to protect the armpits reveal a correction within the underdrawing. Thus on the right fine pale lines envisage a smaller shield shape with a frame and a second fastening loop higher up. In the painted version, the smaller form was used, and the position of the loop was moved again (a position that was changed during the painting process can also be ascertained in the other loop, drawn only once, of the shield plate protecting the other armpit). In addition, while the painted version retained the basic forms of the upper body with the girdle and the cloth garment and mail shirt beneath, there were major changes to the remaining composition. The picture as painted has a brigandine of costly material, whose hems, girdle and breast cross are richly decorated with pearls and gemstones. At the same time, the bow which in the drawing was presumably intended to fasten the right armpit protection was omitted, as was the planned hem decoration of the garment that can be seen emerging from beneath the shoulder plate.