Abstract: A sheet of material having a succession of spaced alternate protrusions and recesses, the walls of each such protrusion and recess being composed exclusively of non-rectangular elementary surfaces joining each other in an undulatory manner at the ridges of said protrusions and recesses along a single line having a plurality of points at which it changes direction to form a plurality of undulations and at each of which points border lines of at least four of said elementary surfaces converge, said surfaces being ruled in a direction extending from one ridge line to another, the sum of the angles formed on said surfaces between said border lines at each of said points being equal to 360.degree..