Mattress coils, also known as mattress springs, are coil springs used in a mattress. Coils are primarily used in the core (support layer) of innerspring mattresses, which is their original use. In recent years, small "micro-coils" are being used in the upholstery (comfort layer) of mattresses, primarily with a coil core ("coil-on-coil" construction), but sometimes with other core types. Mattress coils were introduced in the mid-late 19th century, and remain popular in the 21st century, particularly in the United States. Types There are four types of mattress coils. A key desideratum is "response range", meaning the change in firmness as the spring is compressed – initially soft, to conform to the body, then hard, to provide support. In increasing order of response range and cost, the types are: 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. 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 helicals, these coils form the simplest innerspring unit, also referred to as a Bonnell unit. Steel coils make innerspring mattress construction responsive, but they also don’t offer a whole lot of pressure relief like memory foam does. Offset coils are an hourglass type 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 (Left Facing Knot) coils are an offset coil with a cylindrical or columnar shape. Marshall coils, also known as wrapped or encased coils or pocket springs, are thin-gauge, barrel-shaped, knotless coils individually encased in fabric pockets—normally a fabric from man-made, non-woven fiber. 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. This allows them to react to pressure independently instead of all together, which minimizes movement and allows for a