board's underside. In some rocker-roller boards, the fulcrum (a cylinder or mainly a cylinder) isn't constrained by the board (except by their friction), and in most rocker-rollers the cylinder is constrained by the board in any of five ways (a different number and combination of those ways in each type of rocker-roller) that are described below, in this article's "Rocker-Roller Boards" section. Positions other than standing are also used, in order to work on particular muscles and skills. For better foot traction, the stood-on surface of most boards is manufactured with an unsmooth texture: for plastic models, in the molding; for wooden models, with grip tape or rubber. A smooth surface under the feet or shoes can cause a user to slip off a balance board and fall. Wobble boards are the only type of balance board that is commonly made of plastic. Being no longer than their width, they don't need to be as strong and warp-resistant as other balance boards. Types There are more than a hundred models of balance boards on the market in the United States. Each of them is a version of one of about fifteen types of balance board. Each of these models and types can be classified as one of five basic types of balance board according to two binary parameters: whether its fulcrum is attached to the board and whether the board tilts in only two opposite directions (left and right or forward and back) or in every direction (360 degrees). More specifically: In other words: A rocker-roller board is a rocker board whose fulcrum is a separate piece. A sphere-and-ring board is a wobble board whose fulcrum is a separate piece. A wobble board is a rocker board that tilts toward 360 degrees. A sphere-and-ring board is a rocker-roller board that tilts toward 360 degrees. A spring board rests on compression springs that tilts towards 360 degrees Those five analogies are not precise definitions. They ignore some details of models' structure. Rocker A rocker board is the most basic and least challenging type of balance board. It is a flat board with a fulcrum attached to the board's underside. In some models the fulcrum is perpendicular to the board's length and in the other models the fulcrum is two rockers that are parallel to each other and parallel to the board's length, one in front of the person who is standing