Abstract: A construction system comprises modules of polygonal shape including elongated rectangles, squares, triangles and trapezoids of stiff sheet material formed with integral interlocking tabs along their margins to permit them to be easily and firmly secured to each other to form three dimension structures. Some of the modules also incorporate interlocking slots for securement to each other. This system is especially adapted to the construction by children of an infinite variety of large structures such as playhouses and the like of sufficient size to permit children to play in them.