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There are 3 basic types of innerspring mattresses: the Bonnell, the continuous wire unit and the pocketed coil or Marshall Unit. The original and most common coil in an innerspring mattress is the Bonnell Coil, manufactured by Stearns and Foster. This coil features an hourglass shaped wire wrapped with knotted ends around the top and bottom portions.
The offset coil, which evolved from the Bonnell, replaces the bottom portions of the wire coils with flat sides which facilitate better hinging action when placed within a spring unit. The open end offset coil, used by both Spring Air and Sealy in their innerspring mattresses, uses an extra "turn" of working wire to produce greater spring action on each end.
The pocketed coil or Marshall Unit, manufactured by Simmons Beautyrest, uses individual barrel-shaped coils attached to a perimeter border rod and encased like pockets in a panel of cloth. This process produces an innerspring mattress that conforms to the angles and contours of the body and reduces motion transfer.
Serta's Continuous Wire Unit is technically not a coil system, because it contains rows of single wires that twist and turn the entire length or width of the innerspring mattress.
Innerspring mattress support and durability is influenced by both wire gauge and coil count. Manufacturer test results measure the load deflection, edge and center support of innerspring mattresses, but performance is always the best way to judge the overall quality of an innerspring mattress.
