It is the upper element that allows openings to be created in the walls to form doors, windows or porticos. For a lintel whose ends rest on the vertical elements whose clearance spans the lintel, the greatest stresses occur over the central cross section. Bending causes the upper part of the lintel to be compressed and the lower part to be in tension. Rigid materials such as stone (mass concrete and other ceramic-type materials) withstand tensile stresses worse than compression stresses, so the most common problem in stone lintels are cracks that arise from the bottom of the stone. the underside and progress to the top. Reinforced concrete corrects the problem with steel reinforcements at the bottom that absorb tensile forces. Hence, with this material, lintelled structures (of beams and pillars) are now common, without any of the problems that structures of this type had before, with much larger free spans.