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 . Balls must weigh between . 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. Tournaments that are played under the ITF Rules of Tennis must use balls that are approved by the International Tennis Federation (ITF) and be named on the official ITF list of approved tennis balls. Manner of play Court Tennis is played on a rectangular, flat surface. The court is 78 feet (23.77 m) long, and wide for singles matches and 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 cord or metal cable of diameter no greater than . The net is high at the posts and high in the centre. The net posts are outside the doubles court on each side or, for a singles net, outside the singles court on each side. 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. Tennis is unusual in that it is played on a variety of surfaces. Grass, clay, and hard courts of concrete or asphalt topped with acrylic are the most common. Occasionally carpet is used for indoor play, with hardwood flooring having been historically used. Artificial turf courts can also be found. Lines The lines that delineate the width of the court are called the baseline (farthest back) and the service line (middle of the court). The short mark in the centre