Abstract: A grid structure for engaging and supporting a plurality of coil springs in predetermined spatial relationships so that spring units and completed box spring foundation units can be produced having different numbers of coil springs and consequently different degrees of firmness. The grids are constructed in sizes approximating the dimensions of standard size box spring foundation units, and are formed from longitudinally and transversely extending groups of wires defining a plurality of generally rectangular seats arranged in rows and columns. The top coils of different numbers of coil springs can be engaged with and retained on selected seats in each row to provide the desired degree of firmness of the completed unit. The top coils are restrained against longitudinal and transverse shifting movement relative to their seats and the dimensions of the seats and top coils are such that the top coils can be mounted closely adjacent to each other without overlapping.