CanDo Inflatable Exercise Ball Stabilizer Base provide extra protection and stability for the ball during use. The base also helps prevent the inflatable ball from rolling away when in storage. The small plastic base fits 45 and 55 cm balls, 65 cm and 75 cm balls (sold separately). Stabilizer Base - Small for 45 cm and 55 cm balls and Medium for 65 cm and 75 cm balls