Some are so thick that they must be placed on special low profile frames. Many pillow top type mattresses, in particular, are so thick that it is difficult to find sheets that will fit properly and some persons find it difficult to get into and out of bed. Most mattresses offered today have matching layers of materials on each side, but occasionally you may come across products that do not. Since turning the mattress is necessary for maximum durability you should avoid these one sided products. This part of the mattress probably causes the most confusion among mattress shoppers.
Most mattress makers have a variety of bonnell units in their lines.
The bonnell spring is characterized by its heavy gage wire and knotted ends which wrap around the springs at the top and bottom. One piece of wire spirals up, over, and down to form the coils. The lighter wire and fewer turns are compensated by using a higher coil count even though there may be less steel used than in bonnell types. This Marshall type coil is usually not tempered and of a very thin gage wire. Coil counts are very high due to the relatively weak spring structure.