<P> Tennis balls were originally made of cloth strips stitched together with thread and stuffed with feathers . Modern tennis balls are made of hollow vulcanized rubber with a felt coating . Traditionally white, the predominant colour was gradually changed to optic yellow in the latter part of the 20th century to allow for improved visibility . Tennis balls must conform to certain criteria for size, weight, deformation, and bounce to be approved for regulation play . The International Tennis Federation (ITF) defines the official diameter as 65.41--68.58 mm (2.575--2.700 in). Balls must weigh between 56.0 and 59.4 g (1.98 and 2.10 oz). Tennis balls were traditionally manufactured in the United States and Europe . Although the process of producing the balls has remained virtually unchanged for the past 100 years, the majority of manufacturing now takes place in the Far East . The relocation is due to cheaper labour costs and materials in the region . </P> <P> Advanced players improve their performance through a number of accoutrements . Vibration dampeners may be interlaced in the proximal part of the string array for improved feel . Racket handles may be customized with absorbent or rubber - like materials to improve the players' grip . Players often use sweat bands on their wrists to keep their hands dry and head bands or bandanas to keep the sweat out of their eyes as well . Finally, although the game can be played in a variety of shoes, specialized tennis shoes have wide, flat soles for stability and a built - up front structure to avoid excess wear . </P> <P> Tennis is played on a rectangular, flat surface . The court is 78 feet (23.77 m) long, and 27 feet (8.2 m) wide for singles matches and 36 ft (11 m) for doubles matches . Additional clear space around the court is required in order for players to reach overrun balls . A net is stretched across the full width of the court, parallel with the baselines, dividing it into two equal ends . It is held up by either a metal cable or cord that can be no more than 0.8 cm (⁄ in). The net is 3 feet 6 inches (1.07 m) high at the posts and 3 feet (0.91 m) high in the center . The net posts are 3 feet (0.91 m) outside the doubles court on each side or, for a singles net, 3 feet (0.91 m) outside the singles court on each side . </P> <P> The modern tennis court owes its design to Major Walter Clopton Wingfield . In 1873, Wingfield patented a court much the same as the current one for his stické tennis (sphairistike). This template was modified in 1875 to the court design that exists today, with markings similar to Wingfield's version, but with the hourglass shape of his court changed to a rectangle . </P>

How high is the center of the tennis court net