The Wilson US Open Extra Duty tennis ball is the official ball of the US Open since 1978. The US Open tennis ball is made with exclusive felt from Tex Tech Industries for superior playability, consistency, and durability. The yellow, extra duty felt is made for hard court surfaces. Also available are Wilson US Open Regular Duty tennis ball for clay or indoor courts, Wilson US Open High Altitude tennis ball for all court surfaces over 3500 ft. elevation, or Wilson US Open Grass Court tennis ball with white felt. The Wilson US Open tennis ball is both USTA and ITF approved.   The Wilson US Open Extra Duty case contains 18 cans (4 balls per can - 72 balls per case).   For more information on tennis balls, check out the following blogs: Every Different Type of Tennis Ball Finding the Right Tennis Ball Machine for Your Game Note: Due to the size of this item it does not qualify for free 2nd Day Air shipping. Fee is $14.95 and will be Standard ground shipping.