acute angle w of 85 u00b0 with the lateral longitudinal surfaces 30 1 and 30 2 of the foot segment 20 the head part 21 of the male end segment 16 has a mushroom head shaped recess 38 that is for forming an inner undercut region 36 and that is embodied proceeding from the lateral head surfaces 33 1 and 33 2 of the extension parts 32 1 and 32 2 the lateral head surfaces 33 1 and 33 2 transition to inner lateral longitudinal surfaces 40 1 and 40 2 for forming a sort of mushroom stem for the mushroom head shaped recess 38 the mushroom head of the mushroom head shaped recess 38 has a lateral base surface 41 some of which is linear and parallel to the base 22 and transitions into curved inner lateral surfaces 42 1 and 42 2 without any linear segments so that ultimately a mushroom head is formed minimum widths b 61 and b 62 of the extension parts 32 1 and 32 2 may be found in the region of the mushroom head shaped recess 38 fig2 b depicts the female end segment 16 of the fastener 10 in which are shown the first and only outer transversal undercut region 27 also cited with respect to the complementary embodiment to the male end segment 14 and the innerly disposed transversal undercut region 36 the first transversal undercut regions 26 and 27 extend from the minimum width b 1 of the foot segment 20 of the male end segment 14 to the minimum width b 21 or b.