Tennis is a sport played between either two players (singles) or two teams of two players each (doubles). Players use a stringed racquet to strike a ball, a hollow rubber sphere 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, court tennis or jeu de paume), 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 CE, the game spread first throughout the English-speaking world, particularly among the upper classes. Tennis is now played in the Summer Olympic Games and at all levels of society, by individuals of all ages many countries around the world. Its rules have remained remarkably unchanged since the early 1900s. Along with its millions of players, tennis claims millions of people who follow the sport as spectators, being particularly interested in the four Grand Slam tournaments.
Tennis is played on a rectangular flat surface, usually made of grass, clay, or concrete. The court is 78 feet (23.77 metres) long and 27 feet (8.23 meters) wide for singles matches; for doubles matches, the width is extended by 9 feet (2.74 meters). 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 the court into two equal halves. The net is 3 feet, 6 inches (1.07 meters) high at the posts, and 3 feet (91.4 centimetres) high in the center.
Each of the three primary court types (clay court, grass court, and hardcourt) imparts a different speed and spin to the ball, which affects the level of play for individual players. Some players specialize in certain surfaces on which they are more successful (for example, grasscourt specialists or clay court specialists) or in certain ball-striking techniques (shots or strokes) to which they are best inclined physically.