Abstract: A box-spring core has a first group of first chains extending spacedly adjacent and parallel to one another and a second group of second chains extending adjacent and parallel to one another and transversely of the first chains. Each of the chains has an elongated flexible sheath formed with a plurality of longitudinally spaced, transversely extending, and through-going slits that are each transversely aligned with the slits of adjacent sheaths, a transverse seam adjacent each slit and internally subdividing the respective sheath into a respective succession of longitudinally spaced pockets, and respective springs in the respective pockets each compressible in a transverse direction parallel to the slits. The springs have opposite ends lying generally in respective parallel planes. Each slit has a predetermined length measured perpendicularly between the planes and the combined lengths of a pair of interfitting slits are generally equal to the predetermined height.