Table tennis is an Olympic sport where two or four players hit a ball back and forth to each other, with a racket on a table, in a manner similar to tennis. The rules are slightly different, but the concept is basically the same. Ball spin, speed, strategy and tactics play an important part in competitive table tennis matches. The speed of the ball can vary from slow serves with much spin to smashes that travel as fast as 112.5 kilometers per hour (69.9 miles per hour). The game is played on a 274 cm × 152.5 cm × 76 cm high (approx 9 ft × 5 ft × 30 in high) hard rectangular table with the surface usually colored green, dark blue, or black. A 15.25 cm (6 inch) tall net divides the table in half (similarly to tennis) and is strung to extend 15.25 cm (6 inches) beyond the table on each side. Table tennis requires a large enough room so that players can move freely. In international competitions, the International Table Tennis Federation requires an area not less than 14 m (46 ft) long, 7 m (23 ft) wide and 5 m (16 ft) high. The 4 corners may be covered by surrounds of not more than 1.5 m (5 ft) length. The paddles, also known as bats or rackets, are usually about 10 inches long, with a hitting surface that is approximately 6 in × 6 in (15.2 cm × 15.2 cm), although the rules specify no limitations in size or shape. Modern paddles usually have a thin layer of rubber covering the paddle's striking surface. The rubber may have pimples pointing outwards or inwards, as well as a thin layer of sponge between the plywood center and the rubber surface. Since spin plays a large role in the modern sport of table tennis, the composition of the rubber and the combination of sponge and rubber is designed to maximize the amount of spin and speed a player can impart onto the ball. Other technological improvements include the use of carbon or other synthetic layers as part of the blade to increase the sweet spot or the stiffness of the blade. The ball used in table tennis has a diameter of 40 mm, is made of celluloid, and is completely hollow. A three star rating on a ball usually implies a top quality ball, in relation to its bounce, roundness and their respective consistency between balls of the same make and type. The winner is the first to score 11 points, with each player alternating serves every two points. At 10-10 (or deuce) the players alternate with every serve; the winner is then the first person to gain a clear two points advantage over his opponent. The 11 point game is an International Table Tennis Federation (ITTF) change which occurred in 2001. The 21-point game is still widely played at recreational level. All games played at national level and at international tournaments (ITTF) are played to 11 points in either a best of five (5) games (preliminaries) or best of seven (7) games format (championship matches). Table tennis is very popular in East Asia and is one of the most popular sports in the world in terms of player numbers, as well as one of the newest of the major sports. Ping Pang Qiu is the official name for the sport in the mainland China, Taiwan, and Hong Kong. Takkyu is the official name for the sport in Japan. Tak-ku is the name for the sport in Korea.