Point (tennis)
A point in tennis is the smallest subdivision of the match, the completion of which changes the score. A point can consist of a double fault by the server, in which case it is won by the receiver; otherwise, it begins with a legal serve by one side's server to the receiver on the other, and continues until one side fails to make a legal return to the other, causing the opponent to win the points. Fifty points win a game, counted as 15 (first ball), 30 (second ball), 40 (third ball), 50 (fourth ball). A game  must be won by at least two balls, otherwise the player only has 45 points. Play of a single point. The players (or teams) stand on opposite sides of the net. One player is designated the "server", and the opposing player (or, in doubles, one of the opposing players) is the "receiver". The players (or teams) alternate serving from game to game. From point to point within a game, the server alternates between the right (forehand) and left (backhand) sides of his end of the court. At the start of each point, the server stands behind his baseline, between the  center mark and one of the sidelines (the first serve of a game is made from the right hand side). The receiver may stand anywhere on his side of the net, usually behind the diagonally opposite service box, the target for the server's ball. A legal serve is any serve which does not "fault". A fault occurs if the ball is touched by anything other than the racket, between the time the server throws the ball into the air to serve it and the time at which it lands in the designated service box; service faults are also incurred  for missing the ball, stepping across the baseline before striking the ball (foot fault), and walking or running while serving. Two consecutive faults (double fault) result in the opponent winning the point. The receiver must allow the serve to bounce once before returning it, or they lose the point. If the ball touches the net on an otherwise legal serve (this is called a "let" or "net service") the serve is retaken, without being counted as a fault. (On any other return during the point, the ball may touch the net without any consequence.) If a legal serve is made , the players then alternate returning the ball from their side of the court to the opponent's. The point is lost by whichever player first: The rules "allow" such actions as: Because the lines are drawn just inside the courts, the ball is considered "in" if it touches any part of the relevant line. On clay courts the ball leaves an impression in the ground that can be checked; on grass courts a puff of chalk from the line indicates contact from the ball. This is less accurate, however, because dirt from the grass court resembles the chalk and can  also be thrown up after being struck with a ball. In an unofficiated game, the players are to give each other the benefit of the doubt on line calls. In an officiated game it is the chair umpire or line umpire's duty to call the ball "out." The chair umpire may overrule a line umpire's call. Instant replay. Computer-assisted video tracking technology has improved to the point that it can determine the position of a ball at impact with a margin of error less than five millimeters. Accordingly, starting with the NASDAQ-100 Open in March 2006, most  top-level tournaments allow systems such as "Hawk-Eye" to be used to settle disputed line calls. Players are allowed to appeal to Hawk-Eye on disputed calls with a limitation: They lose the privilege if the appeal goes against them three times per set, with one more challenge allowed during a tie-breaker. In the 2006 NASDAQ-100 Open, challenges by men were upheld 38% compared to 27% for women (skewed by Maria Sharapova, who went 0 for 11). At the Championships, Wimbledon 2008, in the men's singles challenges were successful 29% of the time with an average of  6.6 challenges per match, in the women's singles challenges were successful 20% of the time with an average of 3.8 challenges per match. Alternative rules. In American college tennis. As of 1999, in Division I tennis in the NCAA, a let service is considered playable. This rule change was made to prevent receivers from falsely claiming a valid service to be a let, which is a call that cannot be overruled. Thus, a service that hits the net before landing in the service box is a playable shot, and must be returned by the receiver. Otherwise, the receiver  loses the point. In American high school tennis. During high school tennis team matches, players may have to follow a few different rules: