Inner spring refers to the supportive, coiled metal springs that are found inside a mattress or upholstered furniture. There are several synonyms for inner spring that can be used to describe this type of construction, including pocketed coils, Bonnell coils, and Marshall coils. Pocketed coils are individually wrapped in fabric and operate independently to provide targeted support, while Bonnell coils are the most common type of spring and are shaped like an hourglass. Marshall coils are similar to pocketed coils, but are made from lighter gauge wire for a softer, plusher feel. Other related terms include coil gauge, which refers to the thickness of the wire used in the springs, and coil count, which indicates the number of springs per unit of area.