The Dunlop Grand Prix Extra Duty Tennis Ball series offers the ultimate in performance and durability. A special mix of fourteen ingredients make-up the secret rubber core formula which is combined with the finest woven felt to produce a ball for outstanding performance and feel. The Grand Prix Hard Court balls feature the MaxGlo felt. The MaxGlo felt is 14% brighter than regular cloth, allowing you to see the ball earlier to give you more time for shot preparation. The Extra Duty ball is made specifically to withstand the abrasion from hard courts.