The server has two attempts to get the ball in.
If the ball lands outside the service box or does not clear the net or the net post, it is known as a 'fault'.
If any part of the ball touches the line, the ball is in (as shown above).
After one fault the server may try again. If both tries result in faults, a 'double fault' is called and the opponent wins the point.
With service speeds up to 220 km/hr, it can be difficult for line judges to tell whether a serve is in or out. That is why the electronic eye - often called Cyclops - has been introduced to championship tennis.
The eye consists of a series of horizontal light beams 4cm above the court surface.
The way the ball breaks the beam allows the electronics to determine exactly where the ball bounces.
The rules of tennis still allow the chair umpire to overrule the result of an electronic eye, just as he or she can overrule a line judge.