Our Medium Blue Pre Cut Furniture Balls are slightly smaller than regular size tennis balls for a tighter fit, yet large enough to accommodate all school furniture types (Virco, Arco Bell, Royal Seating, Scholar Craft, and much more). Our Medium Pre Cut Furniture Balls are made with a heavy duty wear resistant felt and a 3.5 mm thick rubber core. All Medium Furniture Balls are Guaranteed to last a minimum of 3 years under normal conditions.