The Penn® Championship Extra Duty Tennis Balls are the official ball of USTA® League Tennis. For use on hard tennis courts, the regular duty balls are built with natural rubber for reliable feel and reduced shock. The interlocked wool fiber composition increases durability for longer wear, while deep-elastic seams offer reduced cracking. This 12 can multi-pack provides the gear you need to be a champion from one season to the next. FEATURES: 12-pack (36 tennis balls) Extra Duty tennis balls for use on hard courts Official ball of USTA® League Tennis and the standard by which all other tennis balls are made Natural rubber construction for consistent feel and reduced shock Interlocked wool fiber for longer wear Controlled fiber release for consistent nap Deep-elastic seams for reduced cracking Model: 521012