on the board and one behind. The ground-contacting edge of the fulcrum is curved in most models and is flat in some models. With one foot placed at each end of the board, the user can tilt it from side to side until the balance point is found and can then either try to keep the board stationary or continue rocking. Rocker boards offer only one degree of movement: part rotation about the longitudinal axis, i.e., banking (left-right tilting). Most rocker boards are made by manufacturers of toys or of gym equipment. Rocker-roller 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). The roller has a different form in different models. Some are a cylinder and some are a cylinder in their midsection that tapers toward the two ends. That tapering enables tricky moves by the rider. How the roller and rocker interface can vary. Rollers may have grooves to fit a guide on the board to keep the roller aligned with the rocker and prevent the rocker from sliding along the roller. Rockers may have guard rails at the ends to prevent the rocker from rolling off the roller. The diameter of the roller of almost all rocker-rollers is between 3.5 and 6 inches at its widest section. The rollers of the rola bolas that circus performers use are usually between 7 and 9 inches in diameter. Wobble The fulcrum of almost all wobble boards is a semi-sphere or smaller spherical cap (or a shape