Abstract: A rectangular box spring assembly of the disclosure includes a grid of crossing wires whose lateral edges have an improved construction in combination with formed wire springs that support the edges of the grid on lateral side members of a lower frame. Each lateral edge of the grid includes a pair of inboard and outboard longitudinally extending wires that are of a heavier gauge than the other wires of the grid. Each formed wire spring on the side frame members includes a pair of fishmouth sections extending upwardly from the frame and a connecting section that interconnects the fishmouth sections and supports the adjacent grid edge. The connecting section of each spring includes an inboard main portion and outboard end portions connected to the main portion. The outboard heavier gauge wire at each lateral grid edge is secured to the outboard end portions of the spring connecting sections to locate the grid.