Open coil springs are usually about 5-6 coils high. A standard double bed has 325 of these coils, although some manufacturers may use more and, of course, the coil count increases as the mattress size rises to king and superking. The coils are all assembled into a single construction with helical wire. This is a thinner, spiral wire that joins the coils at top and bottom to form several rows. An outer rod is then fitted around the top and bottom of the finished construction to provide additional strength. Once the basic construction is completed, the mattress can be filled and the ticking applied.