The tennis court singles line is the inner sideline: the long line that marks the side edge of the court in a singles match. A ball that touches this line is in. A ball that lands completely beyond it is out, even if it lands in the doubles alley.
Knowing which sideline applies makes close calls easier to judge and helps explain why the same shot can be in during doubles but out during singles. Here’s where the singles line sits, how it affects serves and rallies, and what to look for when a singles match is played on a doubles court.
Where Is the Singles Line on a Tennis Court?
On a standard court, the singles sidelines run lengthwise, parallel to the doubles sidelines. They are the two inner side lines, one on each side of the net. The space between a singles sideline and the outer doubles sideline is called the doubles alley.
The singles court is 27 feet wide and 78 feet long. A doubles court is 36 feet wide, so each doubles alley adds 4.5 feet along the side. In singles, the alleys are outside the playing area.
| Court feature | Singles measurement or rule |
|---|---|
| Singles court width | 27 feet |
| Court length | 78 feet |
| Each doubles alley | 4.5 feet wide |
| Singles sideline | The inner side boundary; its line counts as in |
Is the Singles Line In or Out?
The line is in. In tennis, a ball is considered in if any part of it touches a boundary line. So if the ball clips the singles sideline, even slightly, it is a good shot. If the ball lands in the doubles alley without touching the singles line, it is out in singles.
This applies during rallies and to the serve. For example, a serve that lands in the correct diagonal service box but touches the singles sideline is good. A serve that lands beyond that line in the alley is out. The same landing spot may be legal in doubles, where the alley is part of the court.
On a close call, look at where the ball first touches the ground—not where it bounces afterward. A ball can land out and then bounce back toward the court; that does not make it good.
How the Singles Line Works With the Other Court Lines
Baseline
The baseline marks the back edge of the court. It runs across the court at each end, while the singles sidelines run from one baseline to the other. In singles, the area inside the two singles sidelines and between the baselines is the rally court.
Service line and center service line
For a serve, the ball must land in the diagonally opposite service box. That box is bounded by the net, the service line, the center service line and the singles sideline. A serve that lands long, wide, or in the wrong box is a fault.
The center service line divides the service area into left and right halves. It does not mark the side edge of the singles court; that job belongs to the singles sideline. Keeping those lines distinct is helpful when judging a serve near the corner of a service box.
Doubles sideline
The doubles sideline is the outer side boundary of a doubles court. During a singles match, it does not extend the playing area. The line that matters is the inner singles sideline, and the strip outside it is out.
Playing Singles on a Doubles Court
Many clubs use a court marked for both singles and doubles. In that case, the singles lines are already visible inside the doubles sidelines. Players should use the inner sidelines for singles points and ignore the alleys.
There is one equipment detail: when a singles match is played with a doubles net, singles sticks are used to support the net at the correct singles position. They are placed three feet outside each singles sideline. This helps keep the net at the proper height for singles rather than relying only on the wider doubles net posts.
Before starting a match, it can help to agree on which lines are in, especially on a court with several colors of markings. In a standard setup, the singles sidelines are the inner pair running parallel to the doubles sidelines.
Why the Singles Line Matters Tactically
The singles line shapes both attack and defense. A player trying to move an opponent wide may aim close to the sideline, but a shot too far into the alley is out. The safest target is usually a little inside the line, leaving some margin for error.
That margin matters most under pressure. If you are stretched and hitting on the run, aiming directly for the paint can turn a useful defensive shot into an unforced error. On the other hand, a ball that lands close to the line can force an opponent off the court and open space for the next shot.
For the receiver, a ball landing near the sideline can be hard to judge at speed. Watch the bounce and remember that the line itself is good. If you are unsure, calling a ball out when it touched the line gives away the point; in friendly play, a quick, clear call helps avoid disputes.
Frequently Asked Questions
Is the singles sideline the inside or outside line?
It is the inside line of the two side lines on a court marked for doubles. The outer line is the doubles sideline, and the strip between them is the doubles alley.
Does a ball touching the singles line count as in?
Yes. A ball is in if any part of it touches the singles sideline. The same rule applies to the baseline and other boundary lines.
Is the doubles alley out during a singles match?
Yes. In singles, a ball that lands in the alley without touching the singles sideline is out. In doubles, the alley is part of the court during rallies and serves.
Which sideline should I use for a singles serve?
Use the singles sideline as the outside edge of the service box. The serve must land in the correct diagonal box, and landing in the doubles alley is a fault in singles.
Final Thoughts
The tennis court singles line is the inner sideline and the side boundary for singles play. The doubles alley beyond it is out, while a ball that touches the line is in. Once you can spot that inner line, it’s much easier to judge serves, settle close calls, and understand how singles and doubles courts differ.
