The billiard practice training cue ball is made from polyester resin which provides exceptional scratch and impact resistance, and is precision engineered, ensuring superior playability and durability, measuring 2.25""/57 +/- 0.15 mm in diameter and weighing 6 oz/168 +/- 3 grams. The doule-sided design cue ball can help you learn how to strike the ball in the same spot, and you can know more clearly which point will rotate it, as well as the angle and direction of rotation by observing the impact point.