Abstract: A building block wherein two elongated bars are pivotally connected to each other along their longitudinal edges by a first hinge and at least one end portion of each bar is provided with one or more parts of a second hinge which serves to articulately connect the respective end portion to one end portion of a bar forming part of an additional building block. The axes of the first and second hinges are normal to each other. The building blocks can be assembled into skeleton frames forming part of roof structures, erector sets, decorative arrangements, kiosks and/or other combinations of modules.