The core of the mattress supports the sleeper’s body. Modern spring mattress cores, often called "innersprings," are made up of steel coil springs, or "coils."
The gauge of the coils is another factor which determines firmness and support. Coils are measured in quarter increments. The lower the number, the thicker the spring. In general, higher-quality mattress coils have a 14-gauge (1.63 mm) diameter. Coils of 14 to 15.5-gauge (1.63 to 1.37 mm) give more easily under pressure, while a 12.5-gauge (1.94 mm) coil, the thickest typically available, feels quite firm.
Connections between the coils help the mattress retain its shape. Most coils are connected by interconnecting wires; encased coils are not connected, but the fabric encasement helps preserve the mattress shape.
Here are two basic types of mattress coils:
- Bonnell coils are the oldest and most common. First adapted from buggy seat springs of the 19th century, they are still prevalent in less expensive mattresses. Bonnell coils are hourglass-shaped, and the ends of the wire are knotted or wrapped around the top and bottom circular portion of the coil and self-tied.
- Pocketed / Encased Coils or encased springs, are a component part of a mattress in which each coil is separately wrapped in a textile material. Encased coils may also be generically referred to as Marshall coils or wrapped coils.
Bonell springs are hour-glass shaped, which means their resistance increases with load. They are therefore best suited for firm mattresses.