A racket sport contested on a court by either two individual players (singles) or two pairs (doubles). Players use rackets strung to varying tensions according to the power or control they wish to obtain. Rackets are used to hit a tennis ball, which is made of rubber, filled with air, and covered with felt. Players engage in rallies and aim to hit the ball so that their opponents cannot return it within the boundary lines of the court, are unable to return it altogether, or commit an error. The boundary lines and some rules are different in singles and doubles play.