Conventional Bonnell Coils
The first original mattress coils developed for use in horse and buggys. Unique hourglass shape and can be made with many different wire gauges to make them more or less firm. The reason for the hourglass shape is so special is how the spring interacts with pressure. With its narrow design the middle part of the spring can compress with softer pressure and then the rest of the spring "engages" with greater pressure to provide support. Most Spring Coil Mattresses can feature firmer and softer versions.
Although they are not quite as effective as pocket coils or offset coils the Bonnell coils are a good compromise for their very simple and effective design. Because they are less costly to manufacture and provide good value in comparison to benefits they are very popular and a good choice in lower cost mattresses.