The line that marks the back of the tennis court is called the baseline.
A tennis court has two baselines, one at each end, running parallel to the net. They mark the farthest boundary of the court’s length. Players serve from behind the baseline, and a ball that lands beyond it is out, unless it touches the line itself.
The baseline is different from the service line, which runs parallel to the net but sits closer to it and marks the back of the service boxes. The sidelines determine whether play is in bounds for singles or doubles. In doubles, the court also includes the doubles alleys between the singles and doubles sidelines.
The area between the baseline and service line is often called the backcourt or “no man’s land.” Players commonly use the baseline as a strategic home position, hitting groundstrokes from behind it. A player who specializes in this area is known as a baseliner.