Tennis is a sport played between either two players ("Singles") or two teams of two players ("doubles"). Players use a stringed racquet to strike a ball, a hollow rubber ball covered in felt, over a net into the opponent's court. In some places tennis is still called lawn tennis to distinguish it from real tennis (also known as royal tennis or court tennis), an older form of the game that is played indoors on a very different kind of a court. Originating in England in the late 19th Century, the game spread first throughout the English-speaking world, particularly among the upper classes. Tennis is now an Olympic sport and is played at all levels of society, by all ages, and in many countries around the world. Except for the adoption of the tie-breaker in the 1970s, its rules have remained remarkably unchanged since the 1890s. Along with its millions of players, millions of people follow tennis as a spectator sport, especially the four Grand Slam tournaments.