Rocker-roller boards add a degree of instability to the rocker board that makes them much more challenging for the rider than a rocker board is. Rather than on a fixed pivot, a rocker-roller's board is placed on a cylindrical roller; this fulcrum is a wheel that moves in relation to the ground and in relation to the board. The board's pivot point shifts back and forth as the cylinder rolls beneath it. In almost all models the two flat ends of the wheel/roller are about as far from each other as the width of the board. In most models the axis of the roller is perpendicular to the board's length. Thus, as the rider's weight moves over the roller, the board both tilts from side to side and also slides sideways. In models whose roller can be placed with its axis parallel to the board's length, the board slides and tilts toward the front and back (if the rider's feet are oriented accordingly).