The seams between the two pieces of the core are exposed, but covered with elastic sealant. In order to achieve this, most balls are pressurized and filled with air. Some balls are designed for hardcourt and grass, while others are designed for clay courts. In addition, the rules do not say what material the cover should be, the thickness of the cover, or how tightly or loosely the fibers are woven. Players find it takes more effort or less to generate the same ball speed.
Some balls seem springier and bounce higher.
The cover can seem coarser or thicker. With some balls, the cover fluffs up much sooner during match play, seeming to slow down in flight through the air. Most of the major tournaments are played with different brands of balls, designed especially for the different court surfaces. Inc. All Rights Reserved.