Each pad in the series of 5 Progressive Balance Pads has a different level of "give" and offers a different vestibular challenge. Yellow is the least unstable (easiest), and black is the most unstable (most challenging) surface. The Cando® Instability pads are made from the same foam as the low impact aerobic pads. This combination of foam and geometric cut-out design yields comfort and functionality. Pads can also be used as core-strengthening vestibular seating cushions. All pads measure 20 inches in diameter.