The Bonnell Spring Unit is the most common spring unit and consists of hour glass shaped springs, linked top and bottom by a spiral wire which runs across the finished unit. The unit will then usually have a retaining wire that is clipped around the perimeter of the mattress, top and bottom, called a rod edge. This provides the edge of the mattress with extra support. Economy beds may have a plastic as opposed to a metal rod edge and some have none at all. The thickness of the wire used in the construction of the springs is known as the wire gauge. The higher the gauge, the thinner the wire will be and therefore the softer the mattress will feel. Most Bonnell units use a 13 gauge wire. A few orthopedic models will use a 12 or 12.5 gauge as a thicker wire will make a firmer spring. A standard double mattress will usually contain approximately 288 Bonnell springs. The quality of the mattress will then be dependent on the gauge and the amount of filling used. As well as the quality of the cover, some divans have a double layer of springs which should give a much bouncier feel and be more durable over time. You can also have a spring system call Sleepzone or Miracoil (the famous Silentnight spring system) this is similar to the above however the springs run top to toe and normal have around 600 springs per 5' model.