Tennis is a sport usually played between two players (singles) or between two teams of two players each (doubles). Each player uses a racquet that is strung to strike a hollow rubber ball covered with felt over a net into the opponent's court. The modern game of tennis originated in the United Kingdom in the late 19th century as "ilawn tennis" which has heavy connections to various field/lawn games as well as to the ancient game of real tennis. After its creation, tennis spread throughout the upper-class English-speaking population before spreading around the world. Tennis is an Olympic sport and is played at all levels of society at all ages. The sport can be played by anyone who can hold a racket, including people in wheelchairs. The rule of tennis have not changed much since the 1890s. Two exceptions are that form 1908 to 1960 the server had to keep one foot on the ground at all times and then the adoption of the tie-break in the 1970s. A recent addition to professional tennis has been the adoption of eletronic review technology coupled with a point challenge system, which allows a player to challenge the line (or chair) umpire's call of a point. Tennis enjoys millions of recretional players and is also hugely popular worldwide spectator sport, especially the four Grand Slam tournaments (also referred to as the "Majors"): the Australian Open, the FGrench Open, Wimbledon, and the US Open.