the Canadian doubles style, only in this version, players rotate court position after each game, each player taking a turn at playing alone against the other two. As such, each player plays doubles and singles over the course of a match, with the singles player always serving. Scoring styles vary, but one popular method is to assign a value of 2 points to each game, with the server taking both points if he or she holds serve and the doubles team each taking one if they break serve. Wheelchair tennis can be played by able-bodied players as well as people who require a wheelchair for mobility. An extra bounce is permitted. This rule makes it possible to have mixed wheelchair and able-bodied matches. It is possible for a doubles team to consist of a wheelchair player and an able-bodied player (referred to as "one-up, one-down"), or for a wheelchair player to play against an able-bodied player. In such cases, the extra bounce is permitted for the wheelchair users only. Match play Continuity A tennis match is intended to be continuous. Because stamina is a relevant factor, arbitrary delays are not permitted. In most cases, service is required to occur no more than 20 seconds after the end of the previous point. This is increased to 90 seconds when the players change ends (after every odd-numbered game), and a 2-minute break is permitted between sets. Other than this, breaks are permitted only when forced by events beyond the players' control, such as rain, damaged footwear, damaged racket, or the need to retrieve an errant ball. Should a player be deemed to be stalling repeatedly, the chair umpire may initially give a warning followed by subsequent penalties of "point", "game", and default of the match for the player who is consistently taking longer than the allowed time limit. In the event of a rain delay, darkness or other external conditions halting play, the match is resumed at a later time, with the same score as at the time of the delay, and each player at the same end of the court as when rain halted play, or as close to the same relative compass point if play is resumed on a different court. Ball changes Balls wear out quickly in serious play and, therefore, in ATP and WTA tournaments, they are changed after every nine games with the first change occurring after