Background: The V–Y advancement flap and, more recently, the keystone flap are commonly used to cover skin defects. Both flaps allow for primary closure after advancement by substituting the initial defect for a narrower defect distributed over a greater length. The first objective of this study was to develop a geometrical analysis of the V–Y advancement flap. The second objective was to explain the benefit of using the keystone flap compared to a single V–Y advancement flap. Material and method: A geometrical analysis is proposed using a two-dimensional analysis in which the flaps are assumed to have a rigid-body behaviour. First, in the case of the V–Y advancement flap, a trigonometric relationship is defined between the distance of closure before and after advancement, thus implying the value of the flap's apex angle. Second, by considering the keystone flap as the association of three V–Y advancement flaps, the trigonometric relationship is applied to the keystone flap. Results: In the case of the V–Y advancement flap, the optimal apex angles are between 20° and 60°. At less than 20°, the length of the flap increases in an exaggerated manner. At greater than 60°, the distance of closure, particularly at the apex of the flap where a corner stitch is performed, is greater than the distance of closure of the initial defect. In the case of the keystone flap, the width of the final defect around the flap is clearly smaller and more regular compared to the final defect around a single V–Y advancement flap. Conclusion: The geometrical analysis of the V–Y advancement flap in our description illustrates the major benefit of the keystone flap over a single V–Y advancement flap.