In rocker boards and wobble boards, the fulcrum is attached to the board. In rocker-roller boards and sphere-and-ring boards, the fulcrum is a separate piece from the board. In sphere-and-ring boards, the fulcrum (an inflatable or solid ball) is constrained by a ring on the 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.