Bonnell coils are the oldest and original mattress coil system first adopted in the 19th century. It is also the commonly used coil in mattresses manufactured today. Similar to the offset spring system, bonnell coils consist of knotted, round-top hourglass type tempered stainless steel coils. When laced together with cross helical wires, these coils form the simplest innerspring unit. The spring unit is reinforced with an edge frame to add durability.