Penn QST 36 Felt Red Tennis Balls Ideal for beginning players 8 and under playing on a 36" tennis court, the QST 36 Felt features a low compression core modifying the speed and bounce for greater control. With durable felt and a lively bounce, the QST 36 Felt is perfect for driveway play and outdoor courts. All Penn QST products are USTA and ITF approved and endorsed by the USPTA.