A bed board is a rigid, thin board that is placed between the mattress and box spring to add support and firmness. It is especially recommended for people with back problems.
A bed board keeps the mattress from sagging into the box spring. It makes the mattress feel firmer and more supportive. Lack of back support can cause back aches, so making the mattress more firm can often help alleviate many back problems. Putting a bed board under a sofa sleeper mattress keeps the sleeper from feeling the bar through the mattress.
Some bed boards are hinged, making it possible to store them folded, and are necessary for hospital beds with ends that can be raised. Other bed boards are simply flat pieces of wood, sanded smooth so they do not harm the mattress material. Most bed boards are covered with a canvas bag.
Bed boards are sold on the Internet and sometimes in stores. Medical supply stores usually offer them, as well as stores that sell beds or mattresses.