Knowing the tennis court line names makes it easier to follow the score, judge whether a ball is in, and understand where to stand. The key markings include the baseline, singles and doubles sidelines, service line, center service line, and center mark.
Each line has a job, and some matter only in singles or doubles. Here’s how to identify the markings, what they mean during play, and how players use them to guide serves and groundstrokes.
Tennis Court Lines at a Glance
| Line or marking | Where it is | What it does |
|---|---|---|
| Baseline | At each end of the court | Marks the back boundary of the court |
| Singles sideline | Inside the doubles sideline | Marks the side boundary in singles |
| Doubles sideline | The outermost side line | Marks the side boundary in doubles |
| Doubles alley | Strip between the singles and doubles sidelines | Counts as part of the court in doubles |
| Service line | Across each half of the court, parallel to the net | Forms the back edge of the service boxes |
| Center service line | Between the net and service line | Divides each side into two service boxes |
| Center mark | At the middle of each baseline | Shows where a server must stand on the correct side |
The Main Boundary Lines
Baseline
The baseline runs across the back of each end of the court. It is the far boundary for rallies: a ball landing on the baseline is in, while one landing completely beyond it is out. The baseline is also where the server stands to begin a point.
A short mark called the center mark splits the baseline in two. When serving from the right half of the court, the server must stand behind the baseline and to the right of that mark. For the next serve, the server switches to the left half. The server can move along that half of the baseline, but cannot step on or across the baseline before making contact with the ball.
Singles Sideline
The singles sidelines are the narrower pair of side boundaries. They run from baseline to baseline and define the court’s width for singles play. A shot that lands on a singles sideline is in; a shot outside it is out in a singles match.
Doubles Sideline and Doubles Alley
The doubles sidelines are the two outermost side lines. In doubles, they set the court’s full width. The strip between a singles sideline and its neighboring doubles sideline is the doubles alley, sometimes called the tramline.
The alley is in during a doubles rally, but out during a singles rally. That difference is why players need to adjust their targets: a shot aimed near the singles sideline may land safely inside the wider doubles court, while the same ball would be out in singles.
The Service Lines and Service Boxes
Service Line
The service line crosses each side of the court, parallel to the net. It marks the back edge of the service boxes. On a serve, the ball must land in the diagonally opposite service box—not beyond the service line, in the alley, or on the wrong side of the center service line.
The service line is 21 feet from the net. A tennis court is 78 feet long, so the service line sits well inside the baseline. Once the serve lands legally, the service line no longer limits the rally: players can hit the ball anywhere inside the boundaries for the format they’re playing.
Center Service Line
The center service line runs from the net to the service line and splits each side into a left and right service box. It helps define where a serve must land. A serve that touches a boundary line of the correct box counts as in, including the service line or center service line.
For example, if a player serving from the right side aims diagonally into the receiver’s left service box, the serve can land on any line enclosing that box and still be good. If it lands outside those lines, it is a fault.
Service Boxes
The center service line and service line, together with the net and singles sidelines, form the four service boxes. Each box is 21 feet long and 13.5 feet wide. The doubles alleys do not count as part of a service box, even in doubles.
That last detail can seem odd: doubles uses the wider court for rallies, but the serve must still land inside the narrower service box. Once the return is in play, the doubles alley becomes part of the court.
How Court Lines Affect Play
- Line balls are in. If any part of the ball touches a boundary line, it counts as in. A ball does not need to land fully inside the line.
- Singles and doubles use different side boundaries. The singles sidelines apply in singles; the doubles sidelines apply in doubles. The service boxes stay the same width in both formats.
- The center mark guides the server. It shows which side of the baseline the server must use for that point. The server changes sides after each point.
- The service line matters only for the serve. During a rally, a shot can land between the service line and baseline and remain in play.
These markings also shape tactics. A server may aim close to the center service line to pull a receiver toward the middle, or aim wide to open the court. From the baseline, a player might target a singles sideline to stretch an opponent—or, in doubles, use the alley as a wider target. In either case, the line itself is part of the court, so a precise shot that clips it is still good.
Frequently Asked Questions
What are the lines on a tennis court called?
The main court markings are the baseline, singles sidelines, doubles sidelines, service lines, center service lines, and center marks. The space between the singles and doubles sidelines is called the doubles alley.
Are tennis court lines in or out?
Lines are in when they form part of the boundary for the format or shot in question. For example, a singles sideline is in during singles, while the doubles alley is out. A serve landing on a line that encloses the correct service box is good.
Is the doubles alley used during a serve?
No. A serve must land inside the correct service box, which is bounded by the singles sideline, not the doubles sideline. The alley becomes part of the court for the rally after a legal serve.
What does the mark in the middle of the baseline mean?
That is the center mark. It divides the baseline and helps show where the server must stand. The server must serve from behind the correct half of the baseline without touching the baseline before hitting the ball.
Final Thoughts
Once you can spot the baseline, sidelines, service line, center service line, and center mark, a tennis court is much easier to read. The most useful distinction to remember is that doubles uses the alleys during rallies, but not when judging whether a serve landed in. And whichever boundary is in use, a ball touching its line counts as in.
