The most popular and traditional type of mattress, the innerspring mattress is constructed using a system of metal springs or coils to provide various levels of support and comfort. These coils are cushioned between layers of padding, and should return to their original shape when compressed. There are 4 main types of coils:
Open coil mattresses have hourglass shaped coils, which are attached to surrounding coils with a small wire spiral called a helical. This innerspring mattress design provides good initial support, but wears more quickly than other coils.
Offset coil mattresses have square shaped coils, which provide stronger contouring. The innerspring's offset coil mattresses are also attached to adjacent coils using a helical. Offset coil innerspring mattresses are more durable than open coils, since they are more cylindrical in shape.
Pocket coil mattresses or independent coil mattresses have a wrapped cylindrical coil contained within separate fabric pockets. This design provides the best contouring ability. Each coil is separate so the rest of the bed isn't disturbed by the movement of the coils surrounding it. However, this independent innerspring mattress design means each coil cannot share the weight load, in which case its support integrity will wear over time.
Continuous coil mattress is the term for a row of continuous coils made of a single wire attached to the next row by helicals. By sharing the weight load across several innersprings each spring holds its shape and support integrity longer.
In addition to the type of coil used, the coil gauge affects comfort and support. The coil gauge represents how thick (dense) the coil is; thicker coils provide more resistance and therefore firmer support. With innerspring mattresses the coil gauge is represented as a number; lower numbers represent a thicker, more resistant coil.
The coils are often arranged in specific patterns, or coil configuration, to provide more or less support as required. For example, there are usually more coils in the shoulder-to-knee area, as this is where body weight is concentrated. The innerspring mattress' coil configuration will also affect how the coils react to uneven weight distribution (i.e. one sleep partner is much heavier than the other) or sudden movements. The purpose of the coil configuration is to provide optimal, balanced support when and where it is needed.
As a general rule of thumb, the bigger the size of the innerspring mattress, the more coils it should have. Thus, a double sized mattress should have at least 300 coils, a queen size should have 400 coils and a king size should have 480 coils or more.