In Tennis, What Baseline Is?
In tennis, the baseline defines the court’s back boundaries. A full-sized tennis court’s baseline stretches 36 feet along the short sides of the court’s rectangular shape in doubles tennis. To form this rectangle, the baselines meet the sidelines, which are each 78 feet long.
The doubles alleys are not used in singles matches, so the sidelines form a narrower court with a 27-foot baseline. The thickness of all Tennis Court lines, including baselines, must be at least 2 inches. Baselines can be as thick as 4 inches.
There should be enough room for players to move around behind each baseline. This space should have a minimum of 21 feet between it and the wall.

Uses Of Baseline In Tennis
On any tennis court, the baselines are two of the most important lines. They perform basic functions that are critical to the game’s success. Each point, for example, begins with a serve from behind the baseline.
When a ball is hit beyond the baseline, it is considered “out.” Landing the ball as close to the opponent’s baseline as possible without going over it is a common tennis strategy.
This shot is difficult to return because a successful return shot necessitates the returner going backward.
On their respective sides of the court, players are free to step on or over the baselines as they please. This does not apply to serves, which must be delivered entirely from behind the baseline.

The server is not permitted to touch the baseline or the court in any way, and if he or she does, it will result in a foot fault.
Why Is The Tennis Baseline Thicker?
Lines EXCEPT the baseline must be no more than 50 mm thick, but the baseline can be up to 100 mm thick, according to tennis rules.
Informal studies showed that if the line was thicker, it was easier for linespersons AND players on the other end of the court to see it, so more and more of them were widened.