Innerspring Mattresses contain springs that give the bed not only support, but “springiness”. To begin with, quality innerspring (called coils) are usually tempered with heat or electricity. Doing this makes the coils more likely to return to their original shape. Another added quality feature to look for is whether the coils are plastic-coated, which will make them more resistant to wear and corrosion.
Some sales people will try to sell you on an innerspring mattress based on the number of coils it contains. The truth is, however, that the number of coils an innerspring mattress has really doesn’t affect how firm or soft it is. What does determine the firmness is the gauge, or thickness, of the metal coils used. The thicker the coils, the more firm the mattress will be. When asking about coil gauge remember that the lower the gauge number, the thicker the wire is. For example, a 12-gauge wire is thicker than a 14-gauge wire.
There are five different types of coil systems used in innerspring mattresses: Continuous, Karr, Marshall, Knotted Bonnell and Knotted Offset. A continuous coil system is fashioned from a single strand of steel wire strung throughout the mattress. This gives a mattress a lot of strength and makes it quite firm. The Karr systems consists of hour-glass shaped coils with the end left unknotted. The main advantage of Karr coils is that each coil moves independently, such as you would find in a pocket coil mattress. This type of coil system minimizes vibrations from movements. The Marshall system uses very light wire because the mattress contains many coils. Because of the sheer construction and material costs, Marshall coil bed systems are very expensive and are rarely used in mattresses made today. The knotted Bonnel system, like the Karr, consists of hour-glass shaped coils, but each coil has five turns. The Knotted offset is similar to the Bonnell but the difference comes in that part of the top and bottom wire sections are squared off to fit together in a parallel fashion on the top surface. This helps prevents the mattress from sagging.
To top it all off, it is typical for an innerspring mattresses to have a layer of upholstery, latex or memory foam on top. These types of mattresses are called Pillow-Top or No-Flip. It is this top layer that gives an innerspring mattress its soft feel, while the coils below give the support. When you sit on an innerspring mattress you should never be able to feel the springs.