Along representative cross sections of the Apennines and the Northern Barbados accretionary prisms, we measured the area, the d¿collement depth, the angle α of the upper envelope and the angle ¿ of the dip of the regional monocline. The continental sections of the Apennines accretionary prism have a deeper d¿collement than the oceanic sections of the Northern Barbados, 6--10 km depth and <1 km depth, respectively, because the sediment pile is thinner on the incoming Barbados plate and its denser oceanic structure is more easily subducted. Considering the frontal 50 km, the Apennines have an average cross-sectional area of 500 km2 and the Northern Barbados Ridge of 100 km2. The total area is a function of the depth of the d¿collement plane. Therefore, at a given amount of subduction, the deeper the d¿collement depth is, the bigger the area of the wedge will result, assuming negligible compaction and erosion. As a consequence, the larger area/volume and higher elevation of the Apennines with respect to the Barbados is determined by the Apenninic deeper d¿collement. Despite these differences, the geometry of both d¿collements is, in some cases, comparable, in particular, close to the boundary between the crystalline crust and the sediment pile, where the main density and strength contrasts are concentrated. Variations in depth of the d¿collement occur moving along strike in both accretionary prisms. The geometry of the prisms is further controlled by the different values of α and ¿, their sum, and the distance of the accretionary prism relative to the subduction hinge.