Abstract: A modular construction element for building a pigeonhole wall has planar front and back faces flanking a central plane. A rectangular back panel has upper and lower edges bridged by side edges and is formed along the full length of its lower edge with a 45.degree. chamfer. The front panel is subdivided at a central line intermediate the side edges of the back panel into a right section and a left section. The right section has edges coextensive with the corresponding edges of the back panel. The left section is offset in the direction of the line from the right panel to form a recess and a projecting tab. Such elements can be assembled together without the aid of tools or fastening means to form a vertical wall of horizontally throughgoing pigeonholes of diamond shape.