This is the type of mattress that most people are familiar with. Generally speaking the springs are all linked together as a continuous unit with a rigid steel frame. They tend to be robust and rigid in construction and the better quality types would have a thick insulation pad over the springs to prevent spring feel.
The most usual upholstery is dacron or reflex foam with covers varying from a man made woven stitchbond, through to different grades of Belgian damask or cotton. They may be either turnable or non-turn.
Open coil mattresses are favourites with people looking for a very firm or orthopaedic support.