Abstract: The cellular core structure building element is a three dimensional arrangement of pyramidal elements in a continuous rectangular pattern interconnected at their bases by a base plane panel. The pyramid elements are normally void internally to form a structurally strong, lightweight panel. The common pyramid structure is modified by truncating the top of the pyramid to form a top face and by flattening the side face edges to form edge faces thereby creating a pyramid with a top face, four side faces and four edge faces. For best strength as a construction or building material, two cellular core structure sheets are superimposed one on the other by inverting and offsetting one relative to the other such that when mated the pyramid elements of one sheet fit between the pyramid elements of the other such that mating faces may be attached. Continuous faces sheets may be added to the external faces of the structure.