The official who calls the lines during a tennis match is called a linesperson or a line judge. The linesperson stands on the side of the court and watches the lines to determine whether the ball lands inside or outside of the court. They are responsible for making calls on the service line, the baseline, and the side lines. In professional tennis matches, there are usually multiple linespeople working together to make these calls, and they are supervised by a chair umpire who oversees the entire match.