Abstract: Modular furniture is disclosed wherein a system of tubes, preferably made from laminated paper or paperboard, build a system of rectangular frame-like subassemblies. The fill-in panels used in the subassemblies are of the type of hollow prisms, usually rectangular prisms, open at both ends and provided at the open ends with inwardly rounded side edges compatible in size with the surface of the structural tubes. Several embodiments of the fill-in panels are disclosed inclusive of cardboard blanks from which they may be produced. The invention further advances the art of inexpensive modular furniture and its manufacture.