Used hard tennis balls: 8 DUNLOP FORT and 4 Saint James. These balls were used in lessons once a week for half a year, so the "T" is written on the balls. They are especially recommended for seniors in the intermediate class, for ball feeding for elementary and junior high school students, for serve practice, and for warm-up balls. They can also be attached to the legs of chairs to prevent scratches. They are also the perfect firmness for a back massage.