Tennis Court Lines Explained: Every Line, Named and Defined

A tennis court is bounded by six distinct lines per end — the baseline, singles sideline, doubles sideline, service line, centre service line, and centre mark — and under ITF Rule 12, a ball touching any part of a boundary line is ruled in, not out.

Every one of those lines has an exact job. Missing one — thinking the doubles alley is in play during singles, or not knowing which line a serve has to clear — costs points that have nothing to do with skill. Below is what each line does, its official ITF measurement in both feet/inches and metres/centimetres, and how the rules actually use it during a match.

What are the lines on a tennis court?

A full-size tennis court (78 ft / 23.77 m long) carries six named lines, mirrored on both ends and both sides:

• Baseline — the back boundary, at each end of the court
• Singles sideline — the side boundary for one-on-one play
• Doubles sideline — the outer side boundary, used only in doubles
• Service line — the line 21 ft (6.40 m) from the net that closes off each service box
• Centre service line — the line splitting each side’s service area into two boxes
• Centre mark — the short line at the midpoint of each baseline

Add the net, which physically divides the court rather than marking a boundary on the ground, and that’s the entire vocabulary of a tennis court’s markings.

The baseline: where you serve from and the back boundary

The baseline is the line at the very back of each half of the court, running the full width — parallel to the net and as far from it as any line gets. It is the court’s back-of-court boundary: any ball that lands beyond it, even by a fraction of an inch outside the line itself, is out.

The baseline also governs serving: ITF Rule 1 requires a server to stand behind it, between the extension of the centre mark and the singles sideline. It is the only line players must stand behind for a legal serve, which is why it takes more foot traffic than any other line — and why the ITF allows it to be painted wider than every other line: up to 4 inches (10 cm), versus 1-2 inches (2.5-5 cm) for the rest.

The singles and doubles sidelines: what changes between formats

Tennis uses two different width standards depending on the match type, and both sets of sidelines are painted on every full-size court at the same time:

• Singles sideline: sets the court at 27 feet (8.23 m) wide. This is the only sideline in play during one-on-one matches.
• Doubles sideline: sets the court at 36 feet (10.97 m) wide — 4.5 feet (1.37 m) further out on each side than the singles line.

The strip of court between the singles and doubles sidelines is called the doubles alley (or tramline). It is dead space in singles — a ball landing there is out — but fully in play in doubles, effectively adding 9 feet (2.74 m) of total width to the playing area.

The service line and service boxes

The service line is drawn parallel to the net, 21 feet (6.40 m) from it on each side, spanning between the singles sidelines. Every serve has to clear the net and land inside the service box — the rectangle bounded by the net, the service line, the singles sideline, and the centre service line — or it’s a fault.

Each service box measures 21 ft long by 13.5 ft wide (6.40 m by 4.11 m), since the centre service line splits the 27-foot singles width exactly in half. That gives four service boxes total on a full court: two on each side of the net, one for the deuce court and one for the ad court.

The centre service line: splitting the service boxes

The centre service line runs from the net to the service line, exactly halfway between the singles sidelines, dividing each side’s service area into the deuce court (right of centre, facing the net) and the ad court (left of centre). Server and receiver alternate which box is live point to point, which is why the score is announced before every serve. Under ITF rules the line is fixed at 2 inches (5 cm) wide — thicker than the minimum sideline width.

The centre mark: the smallest line with the biggest job

The centre mark is a short line, just 4 inches (10 cm) long, drawn at the midpoint of each baseline, perpendicular to it. It doesn’t bound any playing area — its only job is marking the exact centre of the baseline so a server knows where the boundary is between serving into the deuce court and the ad court. Like the centre service line, it’s painted 2 inches (5 cm) wide.

Do all lines follow the same width rule? ITF line specifications

Line width isn’t decorative — it’s a hard ITF spec, and it isn’t uniform across the court.

Line	Length / span	Width (ITF)
Baseline	27 ft / 8.23 m (singles) or 36 ft / 10.97 m (doubles)	Up to 4 in (10 cm)
Singles sideline	78 ft / 23.77 m	1-2 in (2.5-5 cm)
Doubles sideline	78 ft / 23.77 m	1-2 in (2.5-5 cm)
Service line	27 ft / 8.23 m (between singles sidelines)	1-2 in (2.5-5 cm)
Centre service line	21 ft / 6.40 m (net to service line)	2 in (5 cm), fixed
Centre mark	4 in / 10 cm	2 in (5 cm), fixed

All court measurements are taken to the outside edge of each line, and every line must be a single colour that clearly contrasts with the playing surface, per ITF Rule 1.

Is a ball on the line in or out?

In. ITF Rule 12 is unambiguous: “If a ball touches a line, it is regarded as touching the court bounded by that line.” A ball only needs the smallest fraction of its surface to touch any part of a boundary line — baseline, sideline, or service line — to count as landing inside it. That’s why “just out” calls on tour are decided by Hawk-Eye tracking to millimetres: the margin is often invisible to the naked eye.

Which lines does singles use versus doubles?

The two formats share the same court but use different width boundaries:

• Singles uses the singles sidelines (27 ft / 8.23 m apart), the baseline, both service lines, the centre service line, and the centre mark. The doubles alley is not in play.
• Doubles uses the doubles sidelines (36 ft / 10.97 m apart) instead of the singles sidelines for the full length of the court — including the returns and rallies, not just serves. The service boxes stay the same size in both formats, since the service line and centre service line don’t change with format.

Net posts move too: for doubles they sit 3 feet (0.914 m) outside the doubles sidelines, while a dedicated singles net uses posts 3 feet (0.914 m) outside the singles sidelines. Both hold the net to the standard 3.5 ft (1.07 m) at the posts and 3 ft (0.914 m) at the centre strap.

How court lines affect play

Understanding the lines changes how you read a match. A serve clipping the outer edge of the service line is a legal ace, not a let. A groundstroke landing on the outer paint of the singles sideline is in during singles, even though it looks close to the doubles alley. And because the baseline is painted wider than other lines, a ball landing on its inner versus outer edge can be the difference between “deep but in” and genuinely out — exactly the kind of call that separates recreational disputes from tour-level Hawk-Eye reviews.

For the full build spec beyond just the lines — net height at the posts, surface types, and clearance space around the court — see our breakdown of tennis court dimensions. To see how the same lines are laid out across hard, clay, and grass builds, check types of tennis courts. If you’re wondering why the service boxes matter so much point to point, our guide on how tennis scoring works covers the deuce/ad court alternation in full. And for how tennis court markings compare with pickleball, padel, and squash, see our racket sports hub.