Made of tempered phosphated carbon steel wire, with diameter 2.2 or 2.0. The tempering process plays a fundamental role, because it strengthens the steel and makes it non-deformable, thus preventing those unpleasant depressions in the mattress surface due a sagging spring system, especially in the areas bearing heavier loads. The springs are held together in line by wire spirals. The spring system acts as the bearing structure of the mattress and the anchoring support of the padded layers. The Bonnel spring system is the most traditional solution: 450 or 650 springs (double bed), depending on the type of springs used. The number of springs used is determined by the size of the external rings. With Bonnel springs it is possible to produce both anatomic mattresses with medium bearing capacity and orthopaedic mattresses with high bearing capacity.