A tennis court can look like a grid of white paint until you know what each line does. The baselines, sidelines, service lines, and center marks set the court’s boundaries and decide whether a shot is in, a serve is legal, or a point is over.
Once you can read the lines, it is easier to follow calls and choose where to aim. Here’s a practical guide to tennis court lines, including how singles and doubles boundaries differ and what the markings mean during a serve.
The Main Tennis Court Lines
A regulation tennis court is 78 feet long. The doubles court is 36 feet wide, while the singles court is 27 feet wide. The lines mark the playing area, and a ball that touches a boundary line is considered in.
| Line | Where it is | What it marks |
|---|---|---|
| Baseline | At each end of the court | The back boundary for rallies and serves |
| Singles sideline | The inner long boundary on each side | The side boundary in singles |
| Doubles sideline | The outer long boundary on each side | The side boundary in doubles |
| Service line | Across the court, between the net and baseline | The back edge of the service boxes |
| Center service line | Down the middle of the service area | Divides each service area into two boxes |
| Center mark | A short mark in the middle of each baseline | Helps define where a server may stand |
Baselines and Sidelines
Baselines
The baseline runs across the back of each side of the court. During a rally, a ball that lands on the baseline is in; one that lands completely beyond it is out. Players often stand behind the baseline to serve, though they can move forward after the ball has left the racquet.
The short center mark divides each baseline in half. It gives the server a visual reference: before striking the serve, the player must stand behind the baseline and within the correct side boundary, which is set by the singles sideline and an imaginary extension of the center mark. Stepping on or over the baseline before contact is a foot fault.
Singles and Doubles Sidelines
Each side of the court has two long sidelines. The inner pair are the singles sidelines, and the outer pair are the doubles sidelines. The space between them on each side is called the doubles alley, or simply the alley.
In singles, the alley is out. In doubles, it is part of the court. That difference can make a shot look wide even when it is perfectly legal in a doubles match. A useful habit when watching is to identify the format first, then follow the relevant sideline.
Service Lines and Service Boxes
The service line runs parallel to the net, 21 feet from it. The center service line runs from the net to the service line and splits the serving area into left and right halves. Together with the net and sidelines, these lines form the four service boxes.
Every serve must land in the diagonally opposite service box. The server begins on the right side of the baseline for the first point of a game, then alternates sides after each point. From the right side, for example, the serve must bounce in the receiver’s right-hand service box as viewed by that receiver.
A serve that clips a boundary of the correct box is good. A serve that lands beyond the service line, outside the box’s sideline, or on the wrong side of the center service line is a fault. If the serve touches the net but lands in the correct box, it is a let and is replayed.
How Singles and Doubles Use the Lines
The main difference is the width of the court during rallies. Singles players use the inner sidelines, while doubles teams use the outer ones. The service boxes do not become wider for doubles: a doubles serve still has to land between the singles sideline and center service line, and between the net and service line.
This is why a doubles rally can use the alley but a doubles serve cannot. After the serve lands and the rally begins, the outer doubles sideline becomes the boundary.
How to Judge a Close Line Call
The basic rule is simple: any part of the ball touching a line counts as in. The ball does not need to leave a visible mark or land fully inside the painted area. If it touches the line, it is good.
On hard courts, players may hear a sharp change in sound when the ball clips a line, but sound alone can be misleading. On clay, the mark left by the ball can help show where it landed. At any level, make calls promptly and clearly; if you are unsure and cannot settle the call fairly, give your opponent the benefit of the doubt.
Using the Lines to Play Smarter
The lines are useful targets, but aiming right at them carries risk. A deep ball toward the baseline can push an opponent back, while a shot toward the singles sideline can pull them off court. Leave a little margin rather than trying to paint the line on every shot.
During a serve, the center service line is a handy aiming reference. A serve toward the middle can limit the receiver’s angle; one toward the sideline can open space for the next shot. In doubles, a return aimed down the middle can also test communication between partners, while a shot into the alley can stretch a team if it lands inside the doubles boundary.
Frequently Asked Questions
Does the line count as in during a tennis match?
Yes. If any part of the ball touches a line that forms the boundary of the area in play, the ball is in.
Are the doubles alleys used in singles?
No. In singles, the inner sidelines mark the side boundaries, so the alleys are out. In doubles, the outer sidelines are used during rallies.
Can a serve land on the service line?
Yes. The service line is part of the service box boundary, so a serve that touches it is good, as long as it lands in the correct diagonal box.
What is the center mark on the baseline for?
It helps the server judge the middle of the baseline and the legal area from which to serve. The server must not touch or cross the baseline before striking the ball.
Final Thoughts
Read the court from the outside in: baselines mark the ends, the singles and doubles sidelines set the rally boundaries, and the service lines and center lines shape the serving boxes. Remember that a ball touching a relevant line is in. With those basics, close calls—and the court itself—are much easier to understand.
