There are several different types of coils in a mattress that offer different types of support:
- Bonnell coils are the oldest and most common. First adapted from buggy seat springs of the 19th century, they are still prevalent in mid-priced mattresses. Bonnell springs are a knotted, round-top, hourglass-shaped steel wire coil. When laced together with cross wire helices, these coils form the simplest innerspring unit, also referred to as a Bonnell unit.
- Pocketed coils, also known as marshall coils, wrapped coils, or encased coils are thin-gauge, barrel-shaped, knotless coils individually encased in fabric pockets. Some manufacturers pre-compress these coils, which makes the mattress firmer and allows for motion separation between the sides of the bed. As the springs are not wired together, they work more or less independently: the weight on one spring does not affect its neighbors. Even after wear, two people side-by-side on a pocket spring mattress do not tend to make the mattress sag in the middle. Pocketed coils are used in mattresses to reduce the sensation of movement on a bed.
- Offset coils are an hourglass type of coil on which portions of the top and bottom convolutions have been flattened. In assembling the innerspring unit, these flat segments of wire are hinged together with helical wires. The hinging effect of the unit is designed to conform to body shape. LFK coils are an un-knotted offset coil with a cylindrical or columnar shape. Offset coil systems are known to make less noise because they are constructed with a hinge-like rounded top and bottom. These are often found in higher-end mattresses. Offset coils are designed to hinge, thus conforming to body shape. They are very sturdy, stable innersprings that provide great support. An example of this type of coil design is the Sealy Posturepedic coil system.
- Continuous coils (the Leggett & Platt brand name is Mira-coil) is an innerspring configuration in which the rows of coils are formed from a single piece of wire. They work in a hinging effect similar to that of offset coils. Continuous coils are different in that they are s-shaped, not coiled, and made from one long wire. This can provide a very stable, interlinked coil system that has also been noted as more durable. The largest company using a Mira-coil design is the Serta Mattress Company, though their coil units are supplied from Leggett & Platt.