The present invention relates to the field of modular building systems. Embodiments of the invention include, a modular building system comprising planar sheets of material and moment connections, which are combined with tool-less or limited tool assembly mechanisms and can be used, for example, as spatial partitions, platforms, structural supports, or supports for articles. Specific embodiments of the invention provide structures having a seemingly homogenous, flowing ribbon structure. One embodiment of the invention provides an exhibition support system comprising: (a) at least two planar support members comprising: (i) a top panel with a top planar surface; (ii) a parallel bottom panel with a bottom planar surface; and (iii) multiple walls disposed lengthwise between the panels; wherein the panels and walls define interior hollow cells with inner surfaces and a selected inner cell height and inner cell width; and (b) at least one moment connection with two elongated members: (i) disposed at an angle of orientation relative to one another; and (ii) having a height or width to allow for a pressure fit within one of the cells along the inner cell height or width of the planar member when inserted therein; whereby structural support for the system is internal and is provided lengthwise within the planar support members at the inner surfaces of the cell that interfaces with the elongated members when inserted therein.