Open Coil

Open Coil
Most widely used option, also known as the Bonnell spring.
Springs are arranged in rows and connected to one another, top and bottom, by a spiral helical wire with an outer rod strengthening the perimeter.
There are usually a minimum of 325 coils in a 5ft/150cm King size – but some mattresses may have more.
Come in a choice of tensions, starting from a very firm 12.5 gauge wire.